47 



Spirifera raricosta. (Conrad.) 



Plate 3 A. figs. 5. ^a. ob. 



Delthtris raricosta, (Conrad.) Jour. Acad. Nat. Sci. Phi. yoI. VIII p. 262, pi. 14, fig. 

 18, 1842. 



Delthtris undulatus, (Vanuxcm.) Geology of the Third District of the State of New- 

 York, p. 132, fig. 3, 1842. 

 Spirifera RARICOSTA. (Billings.) Canadian Journal, vol. 6 p. 2591861. 



Description. — Sub-quadrate, sub-semicircular or ovate ; hinge-line 

 equal to the greatest width of the shell, a little greater or less ; dorsal 

 valve usually with five, and ventral valve with six, large rounded or 

 sub-angular ribs ; length of full-grown individuals about one inch ; width 

 equal to or a little greater than the length. 



The dorsal valve is most convex in the middle and more or less flat- 

 tened or concave towards the cardinal angles ; the area narrow, sub-linear ; 

 the beak small pointed and, together with the area, strongly incurved 

 over the hinge-line ; the middle rib, corresponding to the mesial fold of an 

 ordinary Spirifera, is usually very prominent, rounded or sometimes a 

 little flattened on the top ; its width at the front margin, in a specimen 

 fourteen lines wide, is about five lines, and it is well defined and promi- 

 nent all the way to the point of the beak ; the ribs next to it on each side 

 also reach the beak, but the two outer ribs become obsolete 

 on approaching the hinge-line. The ventral valve is most gibbous 

 in the upper half ; the umbo rather small but prominent, and 

 the cardinal angles not flattened ; the area is somewhat variable 

 in its dimensions ; and cannot be seen when the shell has been 

 compressed ; in large perfect specimens it is two lines high at the 

 beak and half a line at the cardinal angles, and slopes outward at an 

 angle of about 100° at its base, but is more or less arched towards the 

 dorsal valve, so that its general direction is more nearly in the plane of 

 the lateral margins ; the beak is small pointed, always incurved over 

 the area ; the mesial furrows and four of the ribs extend quite to the 

 point of the beak ; the mesial furrow in all the specimens that I have seen 

 is broadly rounded, while the lateral furrows are somewhat angular in the 

 bottom. 



The surface is usually covered with small lamellose, somewhat rough 

 ridges of growth ; and in the more perfect specimens with fine imbricating 

 concentric lines, of which there are from four to eight in one line ; all of 

 these are undulated upwards in crossing the ribs. 



In general the area of the ventral valve is only moderately inclined 

 outwards, but it sometimes, in its basal portion, is nearly at a right angle 

 to the plane of the lateral margins. It is always more or less conoave. 



