64 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 

 Fig. 3. Fig. 4. 



Fig. 3. View of a fragment of a dorsal valve from Lockport; showing the cardinal process, 

 socket-plates, etc. 



Fig. 4. Cardinal view of the same, showing the bidentate extremities of the cardinal process. 

 Compare with similar figures of Streptorhynchus crenistria (Phillips) : David- 

 son, Monograph of British Carboniferous Bracliiopoda, PI. xxvii, f. 6 & 7. 



Notwithstanding this deviation from the strict technical description of 

 Mr. Davidson, in the subequal areas and partial or entire absence of a 

 ventral deltidium, considered also in connection with the symmetrical form 

 of the shell, I am unable to see any grounds for a separation from authentic 

 forms of Streptorhynchus. 



Pursuing these comparisons among the species of the older formations, 

 we find that some of those described as Strophomena possess many of the 

 important characters of Streptorhynchus, with the exception that the 

 ventral valve is concave ; which, in lesser degree, is true likewise of Strep- 

 torhynchus ivoohoorthana and S. pandora. The Strophomena defiecta and 

 S. deltoidea, Conrad ; the S. filitexta, S. planumhona and >S^. planoconvexa, 

 Hall ; and S. sinuata of de Verneuil, have the ventral foramen par- 

 tially or entirely closed, and have^ otherwise externally the characters of 

 strias of Streptorhynchus.* The interior of the ventral valve, in two at 

 least of these species, show dental lamellae of moderate strength, which 

 are continued in an elevated border nearly surrounding the muscular im- 

 pression. In the dorsal valve of S. filitexta, we have a bifid cardinal process 

 as in typical forms of Streptorhynchus, but scarcely so much developed 

 as in 8. subplana ; while the teeth-sockets are very shallow, and the socket- 

 plates very little developed. The muscular impression of this valve is like 

 that of typical Streptorhynchus, and unlike that of typical species of 

 Strophomena. 



Fig. 5. 



When we compare the bifurcating cardinal process of Streptorhynchus 

 with that of Strophomena,! we find the latter consisting of a double pro- 

 cess, or of two toothlike processes, which are inclined inwards, flattened, 



* To these may probably be added some other species of resupinate character, 

 t Taking Strophomena rngosa and S. alternata as illustrations. 



