104 



INTRODUCTION. 



Genus — Orthisina, D' Orbigny, 1849. 



Type — 0. ADSCENDENS, Pander, sp., 1830. Int., PI. VIII. figs. 149 — 15G. 



Anomites, Schloth. (part). 

 Obthis (part), of the generality of Authors. 

 Hemipronites and Pronites, Pander, 1830. 

 Orthisina, IfOrhigny, King, M'Coy. 

 Streptorhynchds, King, 1849. 



Animal unknown ; shell semi-circular or subquadrate, with unequally convex valves, 

 external surface variously striated and marked by numerous concentric lines of growth, some- 

 times projecting in the shape of pectinated lamellar expansions; shell structure impunctate ; 

 hinge-line straight, not quite so long as the width of the shell ; area double, larger in the 

 dental valve, straight or bent back at right angles to the valves ; the fissure in both is 

 entirely covered by a convex deltidium, which in the larger valve of a few species is 

 perforated near its extremity by a circular foramen ; the valves articulate by means of 

 teeth and sockets. In the interior of the dental or ventral valve, the muscular impres- 

 sions occupy an area bordered by the raised edges of the dental lamellae, and divided by a 

 short mesial ridge. In the interior of the socket or dorsal valve, a longitudinal median 

 ridge occurs, and the adductors, in the shape of two pairs of transversely oval scars, are 

 placed one above the other on each side of this ridge. 



Obs. Palaeontologists do not seem yet to have completely agreed as to the propriety 

 of separating the shells above described from Orthis. M. de Verneuil' has placed them 

 in several of his sub-divisions of Dalman's genus : they constitute his Eectostriata, with 

 their fissures closed by a deltidium ; but this last-named character, combined with other 

 internal difierences as well as the structure of the shell, renders it very desirable to separate 

 the shells under notice from Orthis proper, which has always the fissure in both valves 

 entirely open, with widely divergent and prominent inner socket ridges, a small cardinal 

 process, and a slight difference in the muscular impressions. Pander^ divided his species 

 of Orthis into several sections, viz. : Orthanibonites, Gonambonites, Hemipronites, and 

 Pronites, the last two or three names referring to forms with a covered deltidium. 



being divided by a more or less elevated mesial ridge or septum. The enormous thickness of the valve 

 may be judged from the following measurements : — 



Length of the ventral valve 

 Width .... 

 Thickness of the valve 



29 lines. 

 ^0 „ 



8 „ 



1 Geol. of Russia and Ural Mountains, vol. ii, 1845. 



2 Beitrage zur Geognosie des Russischeu Reiches, St. Petersburg, 1830. 



