56 PALjEONTOLOGY OF NEW YORK. 



Devonian. In America, Ambocalia uvibonata appears in the Corniferous lime- 

 stone, and its existence is continued under more favorable conditions for devel- 

 opment in the various faunas of the Hamilton group. In the latter faunas is 

 also a larger form which has been described as A. praumbona. Hall, and in the 

 Chemung group A. umhnnata var. gregaria, Hall, is a very abundant shell. The 

 Orthis umbonata was identified in the Devonian of western France (Gahard) by 

 M.ARiE RouAULT as early as 1851,* and has been redescribed and figured by 

 CEhlert in the work already cited. 



There also occurs in the Hamilton shales of western New York a species 

 with spinous exterior {A. spinosa, sp. nov.) ; and in the final appearance of 

 Amboccelia in the Coal Measures, where it is i-epresented by the Spirifer plano- 

 convexus, Shumard, the same condition of exterior occurs. In the latter, how- 

 ever, the surface spines are usually lost, the exterior appearing as in the 

 Devonian species though showing the fine punctation or reticulation of the 

 epidermal shell layer. 



Genus METAPLASIA, nov. gen. 



PLATE XXXIX. 



This name is proposed for the little shell described in 1859 as Spirifer pyxi- 

 datns, Hall.f While it possesses a general spiriferoid aspect in outline, the 

 structure of the hinge and deltidium, the pedicle-valve is the more convex and 

 bears a broad fold, while the brachial valve is fiat or slightly convex over the 

 lateral extremities and depressed medially by a broad sinus. This reversal of 

 the relative position of the fold and sinus is accompanied by some other 

 peculiarities. 



The teeth are stout and unsupported by lamellae ; the posterior extremities 

 of the diductor impressions in the pedicle-valve are deeply impressed and sepa- 

 rated by a short, thick septum. Anteriorly the muscular area is less clearly 

 defined; from its distal margin diverge two ridges which were probably of 

 vascular origin, and a few radiating furrows of similar character are seen on 

 the lateral portions of the valve. 



* Bull. Soc. G6ol. fie France, 2nd ser., t. viii, p. 322. 



t Palaeontology of New York, vol. iii, p. 428, pi. c, figs. 9-12. 



