232 



PALEONTOLOGY OF NEW YORK. 



Shells eloiigate-subtrigonal or subpentagonal in outline, strongly inequivalve, 

 biconvex ; median fold and sinus faint, if at all developed. Anterior margins 

 of contact usually straight, with sometimes a faint fold, at others a low sinus 

 on both valves. Surface with numerous sharp or rounded, simple or divided 

 plications extending from beaks to margins ; cardinal slopes broad and usually 

 smooth. - 



In the pedicle-valve the umbo is elevated, attenuated, mox'e or less incurved, 

 not prone upon the opposite valve. No cardinal area is developed. The del- 

 thyrium is very broad and bears a concave deltidium, which, however, is fre- 

 quently wanting. Teeth small, supported by convergent lamellae which unite in 

 the interior cavity and form a single median vertical septum of variable length ; 

 in the typical species usually extending almost, and sometimes quite to the an- 

 terior margin, and vertically, for fully one-half the depth of the combined valves. 

 The spondylium is very narrow and deep ; combined with the median septum 

 the height of these plates equals fully two-thirds the depth of the valves. The 

 anterior mai'gins of these plates are doubly incurved, the most projecting points 

 being at the base of the septum, and at its line of union with the dental lamellaB. 

 The median septum consists of two vertical lamellae, each continuous with one 

 of the component plates of the spondylium. The spondylium was the seat of 



FIS. 167. PIG. 168. 



Fig. 167. Pentamerus {,Conchidium)[KnighU, Sowerby. A transverse section in the umbonal region. 



s. Septum ol" the pedicle-valve. p. Dental plates forming ventral sponiiylium. 



s'. Septa or the brachial valve. b. Crural plates resting on the septa. (c ) 



Fig. 1B.S. Conchidium laqueatus, Conrad. Transverse section, showing the dellection of the median septum, the 

 deep, narrow spondylium of the pedicle-valve, and the sejita of the brachi.al valve bearing inclined 

 crural processes. (Cj 



muscular attachment, and it bears a series of fine radiating lines along its 

 median portion, and transverse or concentric lines over its lateral slopes ; the 



