DKSCIUI'TIOXS OF I'lIK SPECIES. 101 



Pbntkemites elongatus, /•'. J>. Shumard. 

 (PI. I. figs. 4, 5 ; PI. II. figs. 14, 15 ; PI. XVIII. fig. 4.) 

 Pentremites elongatus, Shumard, Swallow's 1st and 2nd Ami. Report, Geol. Survey Missouri, 



is:.:., pt. 2, p. 187, t. ii. I'. I, 

 Pentremites elongatus, Shumard, Trans. St. Louis Acad. Sri. 1858, vol. i. pt. 2, p. 2 I 1. 

 Pentremites elongatus, Lynn & Casseday, Proc. American Acad. I860, vol. iv. p. 296. 

 Pentremites elongatus, "White, Boston Journ. Nat. Hist, is*;:?, vol. vii. p. -188. 

 Pentremites elongatus, Shumard, Trans. St. Louis Acad. Sci. 18(55, vol. ii. no. 2, p. 384. 



Sj). Char. Calyx elliptical, elongated and attenuated upwards, from one and three 

 quarters to twice as broad as Long ; summit convex and more or less contracted ; base 

 truncate but convex, small, but wider than the summit ; section roundly pentagonal ; 

 periphery as nearly as possible equatorial. Basal plates small, forming a shallow 

 expanded cup; projection of the columnar facet broad and low. Radial plates 

 narrow and very long, quite two thirds the entire length of the calyx ; bodies short 

 and obliquely bent inwards, and subangular in the middle line ; limbs with parallel 

 margins, steep sides, and very obliquely truncated above ; sinuses broadly lanceolate ; 

 lips rather prominent ; interradial sutures in concavities. Deltoid plates acutely and 

 unequally rhombic, and their surfaces concave ; radio-deltoid sutures at about one 

 third the height of the calyx from the summit. Ambulacra convex, extending nearly 

 the entire length of the body, their proximal ends depressed below the edges of the 

 sinuses, but on a level with them distally ; lancet-plate as wide as, if not wider than, 

 the combined side plates on each side ; ambulacral grooves wide and shallow, but the 

 lateral grooves slightly oblique ; side plates oblong, forty to fifty in number. Three 

 hydiospire-folds on each side. Spiracles oval, but often in pairs, and separated by 

 strong septa. Mouth small. Ornament consists of fine lines parallel to the margins 

 of the plates. 



Remarks. The division of the four regular spiracles into eight apertures does not 

 appear to be a constant character. We figure a specimen from Mr. Wachsmuth's 

 collection (PI. I. fig. 4) in which the spiracles are single, and the National Collection 

 contains a similar example, as also one with double spiracles (fig. 5) like those of 

 P. Burlingtonensis. It is noteworthy that these two species are the earliest known 

 representatives of the genus, and that they approach Mcsoblastus in the division of 

 the spiracular openings. The details of the summit structure were not given in 

 Shumard's original account of the type, nor by any later writer, but the relations of 

 the pinnules were described by Dr. C. A. White iu 1863, as we have remarked in 

 our observations on the genus Pentremites 1 . 



Two well-marked varieties can be distinguished amongst the individuals of this 

 species, but there are numerous connecting links between them. 



1 See antea, p. 153. 



T 



