BRACHIOPODA. 109 



Furthermore, from Schizocrania to Q^hlertella and Schizotreta, there is a 

 gradual increase in the degree of depression of this apical area. In the first 

 genus this is very slight, the part often appearing to be in the plane of the 

 valve ; in Trematis the entire subapical area is depressed without any marked 

 development of this feature, while in (Ehlertella pleurites we find it to be ex- 

 ceedingly depressed, so that if the outer margins of the slit were united, a 

 broad tubular passage to the interior would remain. 



In general effect, there is a striking resemblance in the external appearance 

 of the pedicle-area in Orbiculoidea aiid Schizotreta, and in Schizambon. This 

 resemblance is, however, superficial, and can not be allowed to bring the ex- 

 tremes of development along different lines into a very close relationship. 



In Schizambon (S. Jissus), the pedicle-groove lying in front of the beak, is 

 crossed by the interrupted growth-lines of the shell-surface, while in Orbicu- 

 loidea, the tripartite subdivision of the groove, never encroached upon by the 

 concentric growth-lines, invariably maintains its posterior position. 



In DisciNiscA, the same tripartite division of the pedicle-area gives evidence 

 of relationship derived from Orbiculoidea, although the foramen has changed 

 from an oblique to a vertical position ; while, in Discina, the tubular character 

 of the foramen is retained at the expense of the subdivision of the groove and 

 has become inverted in its direction, so that the pedicle is protruded posteriorly 

 instead of anteriorly or toward the apex as in Orbiculoidea. The evidence 

 afforded by the developmental stages of Orbiculoidea demonstrates con- 

 clusively the primary taxonomic position of Schizocrania in this group. 



Wherever there occurs a complete atrophy of the pedicle, it necessarily 

 induces an important modification in the interior of the shell. In the genus 

 Crania, where the whole function of the pedicle is effected by the substantial 

 attachment of the lower valve, the opening and closing muscles are allowed 

 greater freedom of action and their attachment to each of the two valves is 

 of so similar a character that it is frequently a difficult matter to determine, 

 from these features alone, whether a given valve is the upper or the lower. 

 The arrangement of the muscular bands is essentially as in the discinoids, with 



