158 SYNOPSIS OF THE CHARACTERS OF THE 



the A. raricosta, Phil., by the ridges extending only halfway to the beaks on the ventral valve, and its very 

 small size. Length two and a half lines, width three lines. 



Atrypa ventilabrum. Phil. sp. 



Terebratula ventilabrum. Phil. Geol. York. 



Sj). Ch. — Deltoidal, gibbous ; about five distinct, rounded plaits abruptly raised with the margin in front, 

 about ten distinct, rounded, lateral plaits. 



The great number and distinctness of the lateral plaits distinguishes this from A. sulcirostris, to which it 

 is otherwise similar. The margin of the present shell is usually sharper, and the valves more depressed than in 

 that species ; the front is frequently much produced ; the lateral plaits are smaller than the mesial ones, but are 

 perfectly distinct, and continued nearly to the beak. Length nine lines, width one inch. 



Atrypa virgoides. M^Coy. (PL XXII. fig. 21). 



Sp. Ch. — Longitudinally oval; width two-thirds the length; evenly convex; front narrowed, straight; 

 beak of the dorsal valve small, rounded, very slightly incurved; surface concentrically imbricated. 



This is distinguished from the A. viri/o, Phil, which it most nearly resembles by its imbricated surface, it 

 is also considerably larger than the South Devon shell ; it diifers from the A. hastata in the rounded sides of 

 the beak of the dorsal valve and its being very slightly incurved, as well as in its narrow front, and imbricated 

 surface. Like many others of the smooth, or nearly smooth Bruchiopoda, it appears strongly costated longitu- 

 dinally when the external shell is removed. Length one inch four lines, width eleven lines. 



Seminxila. ilf' Coij. 



Gen. Ch. — Shell small, subpentagonal ; smooth, or slightly plaited at the margin; beak of the dorsal valve 

 small, with a minute perforation ; no dcltidium. 



The species of this genus are all small ; nearly smooth shells ; the margin frequently indented, but no dis- 

 tinct plaits on the surface ; the outline is usually more or less pentagonal ; the beak has a very minute foramen 

 for the passage of the muscle of attachment, but there is no deltidium separating the foramen from the hinge. 

 The genus is peculiar to the Palasozoic rocks. 



Seminula PENTAHEDRA. Phil. SP. 

 Terebratula pentahedra. Phil. Geol. York. 



Sp. Ch. — Pentagonal, depressed; front and sides indented, the angles forming broad undulations on the 

 surface ; length and breadth nearly equal, depth half the length. 



This shell appears to have been gregarious, but was, at the same time, very local in its distribution, rarely 

 occurring, but when it does occur it is usually found in great abundance ; it is of a very regular, pentahedral 

 figui-e, the spaces between the angles concave, with an indented margin. Length six lines, breadth six lines. 



Seminula pisum. M'Coy. 



Terebratula seminula. Phil. Geol. York. 



Sp. CA.— Globular; front margin elevated in the middle, with an indentation on each side. Length, 

 depth, and breadth nearly equal. 



This little species is almost spherical ; the elevation of the front margin does not affect the surface of 

 either valve ; the two valves being evenly and nearly equally convex ; the teeth or indentations in the front 

 margin do not produce plaits on the surface. This species appears to have been gregarious, but very local in 

 its distribution. Length one line and a half. 



