238 
PALAEONTOLOGY OF NEW YORK. 
accord the term a subordinate value on the basis of the extravagantly devel¬ 
oped cardinal area in the brachial valve. (See Plate VII, figs. 25, 26.) 
Hemipronites. The type of structure exemplified by this group is distinct 
in many important respects from that of Pronites ( Clitambonites) adscendens. The 
valves are subequally convex, the hinge-line shorter than the greatest diame¬ 
ter of the shell, the greatest depth of the pedicle-valve is not at the apex; 
the deltidium is apparently not perforated, and the surface is covered by ex¬ 
tremely fine radiating striae. Regarding Hemi¬ 
pronites tumida as the type, the association of 
species will represent a very well defined group, 
which may provisionally be held as of subordi¬ 
nate value to Clitambonites, but which when better known may have to be 
more definitely separated from that genus. Its interior characters, other than 
the dental trough supported by a median septum, are not well understood.* 
The features of Clitambonites are very strongly orthoid. This is seen to 
best advantage in the brachial valve, where the difference from the interior of 
0. calligramma rests principally on the modifications produced by the delti¬ 
dium. The Orthis? laurentina-oi Billings, from the Anticosti group, or Mid¬ 
dle Silurian, is in every respect an intermediate form between Orthis calli¬ 
gramma and Clitambonites. In Billingsella the dental plates do not unite, 
though the delthyrium is completely covered in the pedicle-valve and partially 
so in the brachial valve. The earliest appearance of these features is in the 
primordial species of Protorthis and Billingsella, the former genus being, so 
far as known, without a convex deltidium but having the concave dental trough 
or spondylium developed, though unsupported by a median septum. In the 
genus Polytcechia is the earliest known combination of these two features, 
* It is evident that Pander did not regard this first species in his list as a thoroughly normal example 
of the group. He says (p. 74): “Schon durch Pr. oblonga und humilis sahen wir, dass ein Uebergang zu 
den Hemiproniten Statt fand, ein anderer geschieht durch Hemipronites tumida, bei welchem die Riicken- 
flache noch ziemlich hoch hinaufragt, allein nicht riiehr die ^hochste Spitze der Oberschale bildet, letztere 
wolbt sich schon vollkommen, und der aussere Ansehen ist doch noch das einesProniten.” Probably a more 
typical example of his twenty-one species would be H. alta, pi. xxiii, fig. 6, or H. sphcerica, fig. 7. 
De Verneuil, in the Gdologie de la Russie, etc., p. 205, referred nineteen of these species to'the Orthis hemi¬ 
pronites of von Buch, 1840, a name which of course has no value if founded on any of Pander’s species. 
Figs. 9, 10. Hemipronites tumida 
