Description:
 Female:  The short, somewhat slender body tapers anteriorly and posteriorly.  Its cuticle is marked with transverse striations about one micron apart.  The lateral field originates as a narrow groove at the base of the stylet guiding piece and occupies about one-third of the body diameter in the vulvar region.  Incisures originate in the esophageal region so that four are present at the junction of the esophagus and the intestine.  These continue longitudinally for the entire body length until they disappear on the tail.  The outer two incisures are crenate while the inner incisures are straight with no irregularities.  No cervical papilla was observed.  Phasmids occur about one-half the tail length posterior to the anus.  The unstriated tail tip is rounded.
 The low, flat lip region is only slightly set off from the body contour and bears two striations.  The lateral margins of the lips are rounded.  The lateral margins of the sclerotized cephalic framework extend posteriorly about two microns into the body cavity and are more heavily sclerotized than the rest of this structure.  The stylet is short (14.9-16.5 u) and stout with spherical basal knobs.  The guiding apparatus is 3.3-3.8 u long.
 The dorsal esophageal gland duct opens into the convoluted lumen of the esophagus 2.0 u (1.7-2.2 u) posterior to the base of the stylet.  The anterior portion of the distinct esophageal corpus is more broad than the posterior portion.  The ovoid middle bulb contains a heavily sclerotized valvular apparatus.  The sub-ventral esophageal gland duct opens into the esophageal lumen immediately posterior to the valvular structure.  The nerve ring encircles the narrow isthmus about midway between the middle bulb and the junction of the intestine.  The esophageal glands which overlap the intestine are flattened ventrally, appearing long and narrow.  The dorsal esophageal gland nucleus is slightly larger than the subventral nuclei.  The excretory pore opens through the cuticle opposite or slightly posterior to the junction of the esophagus and the intestine.
 Intestinal cells are filled with refractive granules which are about equal in size and evenly distributed.  The intestine terminates in an oblique rectum which opens by means of a faint anus.
 The single, out-stretched ovary extends anteriorly about halfway to the esophagus.  A single row of oocytes is formed in the ovary except for a double row in the anterior portion in the region of multiplication.  The oviduct opens into a prominent Spermatheca which in turn is followed by a cellular uterus.  The vagina extends transversely about one-half the body diameter and opens by means of a conspicuous vulva, the lips of which do not protrude.  The post-uterine branch is about one and one-half times longer than the body diameter at that point.  The posterior portion of this structure is cellular.
 Male:  Males occur as frequently as females and although smaller and more slender are morphologically similar, particularly regarding details of the alimentary tract.  However, the stylet is shorter, 14.6 u (14.3-15.2 u) and the smaller middle bulb tends to be flattened somewhat anteriorly.
 The length of the single, out-stretched male reproductive organ is 45 to 50 per cent of the body length.  The testis comprises the anterior one-third of the system and contains a multiple row of spermatocytes.  The vas deferens and seminal vesicle contain hundreds of small, spherical refractive sperm.  The paired spicules are 14 u long.  They are arcuate dorsally with two swellings, one anteriorly terminal and the other subterminal.  The gubernaculums is 5 u long.  The bursa encloses the tail tip and extends slightly beyond the anterior ends of the spicules.  Phasmids are located slightly posterior to the middle of the tail.

Type host:  Roots of pasture grasses under natural conditions.  Roots of tall fescue, Festuca elatior L. under experimental conditions.
Type locality:  A.H. Smith farm, Upper Marlboro, Prince Georges Country, Maryland.
Type specimens:  University of Maryland, College Park, Maryland.
Diagnosis:  P. subpenetrans can be recognized by its small size, the peculiar form of the lateral margins of the cephalic framework, the abundance of males, and by the shape of the spicules.

 P. subpenetrans can also be distinguished from P. brachyurus, P. coffeae, P. scribneri, P. minyus, P. steineri, and P. hexincisis by the presence of 2 striations (3 annulations) on the lip region.  From P. pratensis it can be distinguished by the absence of striations around the tail terminus and by the presence of a spermatheca.  P. subpenetrans differs from P. pratensis var. tenuistriatus by the absence of striations around the tail tip, the presence of a spermatheca, the conspicuous cuticular striations, and by its longer stylet.  The cephalic sclerotization of P. subpenetrans resembles that of P. thornei, but it differs from that species by its smaller size, its shorter stylet, its rounded but not bluntly rounded tail, the presence of a spermatheca, and the more posterior position of the vulva.  From P. zeae, P. subpenetrans can be distinguished by the presence of males, its smaller size, and the more posterior position of the vulva.  The presence of only two lip striations, its much smaller size, and the more posterior position of the vulva separate P. subpenetrans from P. goodeyi.  P. subpenetrans can be distinguished from P. vulnus by its smaller size.  P. subpenetrans differs from P. tumidiceps by its tapering body form, the more posterior position of the vulva, and by the abundance of males.  P. subpenetrans most closely resembles P. penetrans, but it can be distinguished from this species by the peculiar cephalic sclerotization, the longer post-uterine branch, the presence of vestigial ovary or uterine tissue posterior to the vulva, and its smaller size.  Only about one-tenth of the females of each species were included within the range of L values of the other species.  The L values of male specimens had a larger amount of overlapping.