238 PALEONTOLOGY OF NEW YORK. 



accord the term a subordinate value ori 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, 



1.1 .. niiiT PIT Figs. 9, 10. Ilcmipromtes iiimifia 



which may provisionally be held as oi subordi- After i'andek. 



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. calligrainma rests principally on the modifications produced by the delti- 

 dium. The Orthis ? laurentina of 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 BiLLiNGSELL..i, 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 thiit Pandku ilid not refj;u-d this tiist b|)ecies iu his list a.s a. thoroujfhly nornuil exiimple 

 of the group. He says (p. 74): "Schon durch Pr. oblonga und humilis sahen wii', dass ein Uebergang- zu 

 den Hemiproniten Statt fand, ein anderer geschieht durch Hemiprotiites tumida, bei welchem die Riicken- 

 flache noch ziemlich hoch hinaufragt, allein nicht raehr die hoehste Spitze der Obei-schale bildet, letztere 

 wolbt sich schon voUkommen, 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. sph(Bi'ica, fig. 7. 

 Dk Verneuil, in the Geologic de la Russia, etc., p. 205, referred nineteen of these species to the Orthishcmi- 

 pronites of von Buck, 1840, a name which of coui-se has no value if founded on any of Pander's species. 



