226 PALEONTOLOGY OF NEW YORK. 



" Shell robust, transverse or elongate, sometimes distinctly triangular and 

 globose. -Valves unequally convex, the dorsal always the deeper. Ventral 

 valve with a sinus to which there is not always a corresponding fold on the 

 dorsal valve. Hinge-line straight; hinge-teeth very strong, resting on a broad 

 hinge-plate. In both valves a small obtusely triangular area, which is higher 

 in the ventral than in the dorsal valve. Both valves with a broad pedicle- 

 passage, never closed by a pseudodeltidium. Sometimes the beak of the dorsal 

 valve is so strongly incurved that its perforation is not visible from outside. 

 On the lateral slopes is a more or less strongly defined pseudolunule.* 



" In the interior of the ventral valve are two long, robust dental lamellae 

 which rapidly converge and unite, sometimes before the bottom of the valve is 

 reached, then forming a low median septum. Their anterior portion is always 

 free while their posterior portions are sometimes coalesced into a single piece. 

 In the dorsal valve there are two short crural plates not rising to more than 

 one-third the height of the shell ; these may remain independent or sometimes 

 unite to form a single piece. 



" The muscles are attached between and on the convergent plates, and, in the 

 dorsal valve, also in front of them. 



" Surface-sculpture more or less finely sieve-like. 



" Shell-structure apparently fibrous. 



" All species are confined to the lower Silurian." 



Type, Porambonites intermedia, Pander.f 



The above diagnosis is that given by Noetling, in an admirable paper on the 

 structure and systematic position of the genus.J Although so old a genus and 

 so abundantly represented in the Silurian faunas of Russia, no satisfactory 

 conclusion as to its generic affinities had been reached until the publication of 

 this author's investigations, to whose figures of the interior structure of the 

 valves, obtained from silicified specimens, the student is referred for illustration 

 supplementary to that given in this Volume. 



* This term is apiilied by Nobtlinq to an area on the cardinal slopes usually delimited by some sharply 

 defined growth-line of an immature stage of development. It designates a feature of slight morphological 

 ▼alue. 



t Beitr&ge zur Oeognosie des russischen Reiches, p. 95, pi. xvi a, fig. 12. 1830. 



} Beitrsg zur Hystematiscben Stellung des Genus Porambonites, Pander ; Zeitschrift der deutschen 

 geologischen Gesellschaft, vol. xxxv, p. 355, pis. xv, xvi. 1883. 



