BRACHIOPODA. 199 



anterior slope characterizing the entire group. The internal casts figured by 

 (EuLERT show less highly developed muscular scars and testaceous thickenings, 

 a somewhat irregularly divided hinge-plate supported by a median septum and 

 slightly developed dental plates. The structure is, in short, very similar to 

 that of WiLSONiA, and there appears to be no good reason for dissociating the 

 shell from R. Wilsoni, inasmuch as the relative depth and size of muscular scars 

 are features of but inferior importance. At all events, the type of structure 

 seems to be the same as that which prevails among the earlier representatives 

 of Camarotcechia. Bayle's species, however, bears no little resemblance to 

 Sowerbt's Terebratula Stricklandi* of the Wenlock fauna, a species finely devel- 

 oped in the Niagara faunas of Indiana, Illinois and Wisconsin. We have been 

 supplied with beautiful internal casts of this species by T. A. Greene, Esq., of 

 Milwaukee, obtained from the dolomites in that vicinity, and these show a 

 peculiar conformation of the hinge-plate, the lateral components of which are 

 divided medially for a portion of their length only, toward the apex the plates 

 being curved upward and uniting, thus forming an arched and hollow process 

 which is, of course, the representative of the cardinal apophysis in Uncinolus. 

 This is a very simple condition of development of this process, and it is inter- 

 esting to find it so early in the history of the group. The figure of the hinge- 

 plate of R.falladosa given by (Ehlert (5/), is similar to the impression usually 

 obtained from internal casts of R. Stricklandi, and it may be found that the 

 French species possesses the incipient cardinal process of that shell ; in this 

 event the term Uncinulina may have a certain value as a distinctive designa- 

 tion for shells in this condition of development, but for the present it seems 

 wiser to include R. Stricklandi within the limits of the genus Uncinulus. 



It is in the fauna of the Lower Helderberg group that the subcuboidal Rhyn- 

 chonellas with a highly developed process, attain their characteristic and most 

 extreme development. Dr. (Ehlert's figures of this process in Uncinulus sub- 

 wilsoni show it to be a simple crescentic apophysis striated longitudinally, but 

 in Rhynchmella mutabilis, R. abrupta, R. vellicata, and R. nucleolata, Hall, of the 

 Lower Helderberg group, there is usually some evidence of a median division ; 



* Not Rhynchonella Stricklandi (Sow.), Schnur, which is a Devonian shell. 



