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 palajozoic 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 palteozoic 

 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 casta of R. loxia, from Charas- 

 chowa, Russia. 



