BY ROBERT M. JOKN'STON, F.L.S. 139 



covered by a pseudo-deltidium. The external surface of the 

 shell is ornamented by a variable number of radiating ribs, 

 which augment in number to a greater or lesser extent, from 

 intercalations at unequal distances from the beaks ; so that from 

 70 to 90 may be counted round the margin of each valve in 

 adult indiN'iduals. The ribs on the fold and sinus are likewise 

 more flattened than on the lateral portions of the shell. The 

 surface is closely and finely reticulated. In the interior of the 

 dorsal valve, under the extremity of the incurved umbonal beak 

 there exists a small cardinal process or muscular fulcram, and on 

 either side are situated the dental sockets. The spiral cones which 

 fill the larger portion of the shell are attached to the extremities 

 of the inner socket-walls. The lamellae, after having converged 

 and given birth to the crural processes, diverge, and form the 

 first of tlie 20 or 22 convolutions of which each spiral is 

 composed. Four impressions left by the adductor muscle are 

 risible in this valve. In the interior of the ventral valve a 

 strong hinge-tooth is situated on either side at the base of the 

 fissure, and is supported by a vertical shelly plate of much 

 strength, but not advancing to any great length, into the interior 

 of the valve. Between these a large portion of the free space 

 at the bottom of the shell is occupied by the adductor and 

 cardinal muscular impressions, which are divided by a blunt, 

 central, longitudinal ridge. The dimensions of one of the 

 largest examples are : — 



Length, 4|- in.; width, 6 in. 1 line ; depth, 3 in. 1 line. 



Spirifera laminosa, M'Coy. 



Transversely sub-rhomboidal ; valves unequally convex, the 

 ventral one by far the deepest. The lateral portions of the 

 shell are regularly curved, forming with the extremities of the 

 hinge-line, acute, but not prolonged cardinal extremities : area 

 large, triangular, more or less elevated, and divided by a fissure 

 of moderate width. Beak small, not much produced above or 

 beyond the level of the area. The mesial fold in the dorsal 

 valve is broad, and more or less elevated without ribs, and 

 corresponding with a deep and rather wide longitudinal sinus 

 in the ventral one. Each valve is ornamented by about 20 or 

 22 narrow radiating ribs, intersected by closely disposed, sharp, 

 oncentrie, undulating laminae. The measurements from two 

 examples have produced — 



Length, 12 ; width, 21 ; depth, 10 lines, 

 jj 8 „ 11 ,, o^ „ 



Spiriferina cristata, var. octoplicata, «7. De C. Sowerhy. 



Transversely sub-rhomboidal, valves about equally convex, 

 and at times rather gibbous ; hinge-liue as long as the greatest 

 width of the shell. Cardinal angles acute or slightly rounded ; 

 area concave, triangular, and of variable width ; fissure partly 

 covered by a pseudo-deltidium ; beak small and incurved. The 



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