BRACHIOPODA. 
179 
small central adductors. The posterior surface about the muscular area is pit¬ 
ted with ovarian markings. 
In the brachial valve there is no cardinal process; the crural plates are 
simple divergent, somewhat expanded on the upper surface but not conjoined 
except where they converge beneath the beak and meet the median septum, 
which extends for about one-half the length of the valve. The crura are long 
and curved upward toward the opposite valve. Muscular area elongate-sub- 
quadrate, with small posterior and large anterior adductor scars. 
Shell-structure fibrous. 
Type, Rhynchonella loxia, Fischer de Waldheim.* Upper Jurassic. 
Observations. It may be doubted whether precisely this combination of 
internal characters exists among the palaeozoic faunas. To the expression 
of so extreme a view we have been led by the fact that of all the 
preparations, natural and mechanical, of the interior structure of these shells 
that have been examined, none show a strict conformity therewith, each 
possessing some variation of considerable significance; a linear or a clavate 
cardinal process; absence of dental lamellae or brachial septum; coalesced crural 
plates or an inter-crural pit. These differentials permit groupings of the 
palaeozoic species among themselves, which do not include the typical Rhyn- 
chonellas. The interior of many of the American palaeozoic species is still 
unknown; the foregoing statement is based upon the representatives of the 
various faunas that we do know, which, indeed, taken together make a major 
percentage of described species. As to exterior characters, the peculiar modi¬ 
fication of form possessed by R. loxia is most rarely met with in palaeozoic 
species, perhaps only in the R. acuminata^ Martin, of the upper Devonian and 
the Carboniferous, and, naturally enough, this species fails to conform in 
internal structure with R. loxia. The modifications of external form, while 
manifestly of subordinate significance, accompany with some persistence the 
variations of the interior. 
* This diagnosis has been derived from excellent exteriors and internal casts of R. loxia, from Charas- 
chowa, Russia. 
