NUCLEOSPIRA 453 



Family MERISTELLID.E 

 Genus NUCLEOSPIRA Hall 



Description. Shells small, subcircular in outline, the valves subequally 

 convex, the hinge-line very short, the cardinal extremities rounded, the 

 mesial sinus of the pedicle valve and the fold of the brachial valve low 

 and ill-defined, usually obsolete except near the anterior margin. The 

 epidermal layer usually, if not always, covered with numerous, closely 

 crowded, fine, short spinules. Pedicle valve with a very low and small 

 cardinal area which is obscured in mature shells by the incurvature of 

 the beak, the delthyrium closed by a pseudodeltidium. Internally the 

 hinge-teeth are prominent, approximate, recurved at the tips and sup- 

 ported by thickened bases but not by dental lamella; the muscular area 

 rather large and flabellate, not sharply defined about its margins, divided 

 longitudinally by a low but distinct median septum which extends nearly 

 to the front margin of the valve. In the brachial valve the cardinal 

 process and hinge-plate combined rises nearly vertically from the bottom 

 of the valve, but just above the plane of the margins of the valve it is 

 abruptly bent posteriorly so that its upper surface is nearly parallel with 

 the plane of the valve and is extended beyond the margin of the valve 

 posteriorly into the umbonal cavity of the pedicle valve ; the crural bases 

 are situated on the vertical face of the plate at the point of recurvature, 

 the crura are straight and slender with a length equal to one-fourth the 

 length of the shell; the primary lamellae of the brachidium are greatly 

 incurved, the spires are made up of from six to ten volutions, their apices 

 directed transversely ; the jugum originates at about one-fourth the length 

 of the lamellas, it is inclined slightly backward, the lateral branches are 

 united and produced into a straight, elongate, undivided stem. The 

 muscular scars are narrow and elongate, ill-defined and are divided by a 

 median septum similar to that of the pedicle valve. 



Remarks. The members of this genus are usually easy to recognize 

 from their external form, viz, the subcircular and nearly equally convex 

 non-plicated valves. In internal casts they may be easily recognized by 

 the form of the shell and by the presence of the elongate median septum 

 in the condition of a median slit, extending nearly to the front of each 

 valve. The minute surface spines of members of the genus are com- 

 monly destroyed in the fossil specimens, but their former presence often 

 may be determined by the presence of the minute papillae-like bases of 

 the spines. 



