BRACHIOPODA. 339 



the size of the ventral and dorsal mantle lobes, seems to be a partial if not 

 sufficient explanation of its existence. As either the presence or absence of 

 this reversal is a normal secondary condition, it is not possible to give it great 

 weight in a broader grouping of the genera, for we find that Strophonella is 

 but a reversed Stropheodonta, passing through similar phases; A.mphistrophia 

 is a reversed Brachvprion, both existing in faunas of the same age, and Stro- 

 phomena is a reversed Rafinesquina, both similarly coexistent. 



With tliis presentation of the subject it seems neither necessary nor desirable 

 to propose any bi-oad division of this group of genera. In 184G King proposed 

 to embrace Stropiiomexa and its allies, in the family Stropbomenid^. The 

 large number of generic values allied to Strophomena, which have been deter- 

 mined since that date, make this comprehensive family divisible ad libilum, sed 

 non in major em Dei gloriam. 



The calcareous fixation of the pedicle-valve to extraneous bodies after the 

 closure of the pedicle-passage and atrophy of the pedicle itself, is repeatedly 

 manifested by these shells. This, as already shown, is a pre-adult condition in 

 Orthothetes, Derbya and Streptorhynchus, the shell becoming wholly free 

 before full growth was attained ; lint in Lept^nisca and Davidsonia the attach- 

 ment was maintained throughout tlie later existence of the shell. 



The impref ions left by the spiral arms upon the interior of the valves in 

 Davidsonia and Lept^nisca, and also observed by Davidson in specimens of 

 Rafinesquina Jukesi and Leptccna rhamboidulis, show a complete correspondence in 

 the direction and curvature of the coils, and we are left to infer that other 

 members of the BmopiroMENiDyE were in agreement with this structure, and, 

 hence, that the arms in their uncalcified condition approached nearer the cal- 

 cified spirals of Konikckikidj; (Coelospira, KoNiNCKiNiA, etc.) than to any other 

 group. 



The condition of the pedicle-passage possessed by these shells is maintained 

 by Chonetes and Productus, without great modification in other respects. 

 Chonetes possesses a marginal row of strong cardinal spines or tubes communi- 

 cating with the internal cavity of the valves. Yet w^e are acquainted with 

 forms (e. g., Anoplia 7iucleata) in which these spine-tubes do not manifest them- 



