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; Amphistrophia 
is a reversed Brachyprion, both existing in faunas of the same age, and Stro- 
piioMENA is a reversed Bafinesquina, both similarly coexistent. 
With this presentation of the subject it seems neither necessary nor desirable 
to propose any broad division of this group of genera. In 184G King proposed 
to embrace Strophomena and its allies, in the family Strophomenidji:. The 
large number of generic values allied to Strophomena, which have been deter¬ 
mined since that date, make this comprehensive family divisible ad libitum, sed 
non in mojorem 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 ; hut in Lept^nisca and Davidsonia the attach¬ 
ment was maintained throughout the 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 
Rajinesquina Jukesi and Leptaina rliomhoidalis, show a complete correspondence in 
the direction and curvature of the coils, and we are left to infer that other 
members of the Rtrophomenidje were in agreement with this structure, and, 
hence, that the arms in their uncalcified condition approached nearer the cal¬ 
cified spirals of Koninckixid^ (Ccelospira, 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 we are acquainted with 
forms (e. g., Anoplia nucleata) in which these spine-tubes do not manifest them- 
