NOTICE OF PALEONTOLOGY OF NEW YORK. 193 



There is also another type usually included under the Genus Pentamerus, 

 which may be separated with advantage. This one is rejsresented by 

 P. reversus (Billings), P. verneuili and P. interpUcatiis (Hall), etc. In 

 these forms the relation of the valves, as in typical Pentamerus, is 

 reversed, and the interior structure presents other important differences. 

 For these I shall propose the name of Anastrophia. 



There is still a farther separation required among the pentameroid 

 forms, or an extension of the characters of the genus. The Pentamerus 

 occidentalis of the Iowa Geological Report presents externally a depressed 

 dorsal valve, with a median fold, on the lower valve; while there is a 

 distinct area bordering the fissure, and this area is vertically striated as 

 in many of the Spirifers. The ventral valve has the trough-like pit, 

 formed by the junction of the lamellae, greatly extended and extremely 

 incurved, and the dorsal lamellae or crura are divergent and present some 

 peculiarities. 



It is doubtless unsafe to base a distinction of genera upon external 

 characters alone ; and even with a partial knowledge of the interior 

 structure, we may be misled, but this group of shells presents itself to 

 us under an aspect that will admit of the following arrangement : 



1. Pentamerus proper, having rotund or gibbous forms, with the 

 ventral valve prominent in the middle, and the dorsal valve flattened 

 or depressed towards the front ; lamellae of the dorsal valve distinct : P. 

 hiightii, P. galeatus, P. pseudogaleatus. 



2. Elongate forms with the valves subequally convex, lobed or sub- 

 sinuate ; internal structure essentially as in P. knif/htii, and of which P. 

 ohhngus^ P. lens? are typical forms. 



3. Forms ovate, more or less rotund, with a sinus on the ventral valve 

 and a mesial fold on the dorsal valve ; internal structure of the ventral 

 valve as in P. kniglitii. Dorsal A^alve with the crura or lamellae of the 

 hinge-plate conjoined so as to form a separate trough-shaped cavity, 

 which unites with the inner surface of the valve ; a narrow area on each 

 side of the fissure, and a flattened space or false area along the cardinal 

 margin of the valve. P. aratus and P . papilionensis are of this type. 

 Genus Pentamerella. 



4. Forms more or less elongate, lobed or with mesial fold and sinus ; 

 hinge with an extended area on the ventral valve : internally a short 

 V-shaped pit in the ventral valve supported by a septum. In the dorsal 

 valve, the crura are free almost or quite from their origin (as in Spiri- 



Cab. Nat. 25 



