EARLY DEVONIC HISTORY OF NEW YORK AND EASTERN NORTH AMERICA 20I 



with reference to the known species of the more southern province because 

 of the frequency with which a given specific group presents the differentials 

 of more than one of the affihated forms. This situation is somewhat further 

 compHcated by the ready passage of more extreme forms of Rhipidomella 

 into the conservative expression of Hipparionyx proximus. The 

 work of BilHngs has not given much aid in the interpretation of these rela- 

 tions. In the lists of fossils cited by Logan, for which Billings was respon- 

 sible, Orthis oblata (a Rhipidomella described from the Helderbergian 

 of New York) was given, but in the Palaeozoic Fossils the only species of 

 this type referred to is Orthis livia which Billings had described in 

 i860 from the Onondaga limestone of Ontario and Hall had subsequently 

 identified in New York [Pal. N. Y. 1865, 4:38]. Billings however gives 

 only a reproduction of his original description and figures and makes no 

 mention of the special traits of the Gaspe shell. After careful analysis we 

 conclude that the more closely related shells of the Rhipidomella type here 

 present are divisible into three groups whose differentials will be presently 

 noted: (i) a relatively small species of the general aspect of R. oblata; 

 (2) a larger form which is in a certain sense an intensified expression of 

 R. musculosa and (3) a conservative expression of Hipparionyx 

 proximus. 



Rhipidomella musculosa Hall 



Plate 42, figures 1-5; plate 43, figures 14, 17, 19, 20, 22 



Orthis livia Billings (in part ? ). op. cit. p. 32 



Orthis musculosa Hall. Palaeontology of New York. 1859. v. 3, p. 409, pi. 



91. fig- T-3 ; 95. fig- 1-7 



Rhipidomella musculosa Hall & Clarke. Palaeontology of New York. 

 1892. V. 8, pt I, pi. 6A, fig. 5 



These shells are conspicuous for their size, averaging on the whole 

 larger than the representatives of the species from the littoral sand deposits 

 of the Oriskany of New York. 



They are strongly developed in all details, the muscular scars of the 

 ventral valve being very large and extending well toward the anterior 

 margin and the median ridge between the adductors is frequently elevated 

 into a strong septum. The pallial region, though narrow, is ridged and 

 pustulose and the shell substance in this valve frequently very thick, the 

 scars being deeply excavated with elevated margins. The dorsal valves are 

 quite convex and have the aspect of dorsal valves of Hipparionyx though 

 less transverse on the hinge margin. From the beak the margins slope 

 away in a characteristic manner. The elevation of the surface is greatest 

 at about the middle and the slope therefrom gentler toward the postlateral 

 margins than toward the front. The crural processes are well developed 

 but the cardinal process is rather short, not projected beyond the hinge 

 margin and its posterior face is not trifid but chevroned. The shell 

 substance is punctate in the inner layers. 



