BRACHIOPODA. 
109 
Furthermore, from Schizocrania to (Ehlertella 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 and 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 
