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PALAEONTOLOGY OF NEW YORK . 
muscular impressions in that genus lying upon the median thickening but in 
front of it in the others. The presence of a median septum in both Obolus 
and Trematis adds to their similarity. The large median scars in Trematis, 
however, are composite, and thus indicate an important difference in the char¬ 
acter of the muscular anatomy of the two genera. In position, if not in func¬ 
tion, the anterior and median members of the great central scars in Trematis 
correspond with the anterior laterals and centrals in the brachial valve of 
Lingula. 
In Schizotreta (=Orbiculoidea, Davidson; see pp. 120, et seq.), we find a gene¬ 
ral correspondence in the character of these impressions, although our knowledge 
of the muscular scars of the brachial valve in that group is limited to a single 
species, S. conica, Dwight, and in this case only to the two large central scars, 
parted by a thickened median area and septum. In the absence of satisfactory 
evidence concerning these important characters in the brachial valve of Orbicu- 
loidea, D’Orbigny, we may assume that their arrangement is indicated by what 
is known in the case of Schizotreta, a group very closely allied in other 
respects. 
In the pedicle-valve of Trematis there is an apparent correspondence in the 
character of the pedicle-opening with the forms which have been referred 
above to Orbiculoidea, D’Orbigny. In the discussion of these fossils it has 
been shown that the aperture is not an oval perforation as in Discinisca, or a 
fissure extending to the margin as in Trematis, but a tubular oblique passage, 
most closely allied to that in Siphonotreta. The homology in this respect 
then becomes remote, though distinctly traceable through the aberrant Distinct 
(CEhlertella ) pleurites, and perhaps is even more direct in the case of Lindstrcem- 
ella. Schizobolus, Discinolepis, Schizocrania and perhaps Kutorgina conform 
with Trematis in having the aperture a radial incision, and as the nature of the 
pedicle-opening must be considered as of radical importance in determining the 
taxonomy of the inarticulate genera, the groups named will fall into close 
contiguity. It has been shown how closely Schizobolus is allied to Obolella 
in the character of its muscular anatomy, and that Kutorgina probably repre¬ 
sents an incipient stage in the development of the Siphonotretoid pedicle-tube. 
