238 PALAEONTOLOGY OF NEW YORK. 



accord the term a subordinate value oil 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) adscendem. 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, 



, 111 c 1 T Figs. 9, ]0. ncmipronitcs tnmMa 



which may provisionally be held as or subordi- Aftei pauuee. 



nate vakie 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 

 O. calligramma 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 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 PaiNueu (li<l not regard tliLs tii'st species in bis list as a tliorougbly normal example 

 of the group. He says (p. 74) : "Schon durch Pr. ohlnnga und humilis sahen wir, dass ein Uebergang zu 

 den Hemiproniten Statt fand, ein andei-er geschieht durch Hemipronites Uimida, bei welchem die RUcken- 

 flache noch zieinlicli hocli liitiaufi'agt, allein nieht raehr die hochste Spitze der Obei-scbale bildet, letztere 

 wolbt sich scbon vollkommen, und der aussere Ansehen ist doch noeh das einesProniten." Probably a more 

 typical example of bis twenty-one species would be ff. alta, pi. xxiii, fig. 6, or H. sphmrica, fig. 7. 

 Ue Veh.neuil, in the Geologic de la Russie, etc., \t. 20.5, referred nineteen of tbe.se species to the OrtMslicml- 

 pnmites of von Bueii, 1840, a name which of course has no value if founded on any of Pander's species. 



