No. 186.] 35 



Genus Eatonia* (Hall, 1856). 



Terebratula, in part, of authors. 



Atrypa : Conrad, Vanuxem, Hall, and Mather. 



Rhynchonella, in part, of authors. 



Shell oval or ovoid, subcircular, elongate or transverse. Valves 

 very unequally convex, witli a strongly developed mesial fold and 

 sinus. Beak of tlie ventral valve perforate. 



The ventral valve is usually nearly flat or slightly convex near 

 the beak, flattened or concave in the middle, with a broad deep 

 sinus extending thence to the front of the shell; the anterior ex- 

 tension being often turned at right angles to the plane of the lon- 

 gitudinal axis. Beak small, elevated and closely incurved over the 

 umbo of the opposite valve, perforate : no area. Dorsal valve 

 convex, often ventricose, with a deep sinus in the anterior margin. 

 Valves articulating by teeth and sockets; the anterior and antero- 

 lateral margins often crenulate or plicate within. The cardino-lateral 

 margins of the ventral valve are usually angularly inflected, and 

 embraced within the edges of the dorsal valve. 



The valves articulate by means of two teeth in the ventral valve 

 with corresponding sockets in the dorsal valve, and a medio-longi- 

 tudinal ridge in the ventral valve which is more or less completely 

 embraced between the deeply bifurcating cardinal process of the 

 dorsal valve, which forms part of the apophysary system. 



The dorsal valve has a prominent bifurcating cardinal process, 

 the branches of which, slightly diverging, form the first or lower 

 crural processes, which are directed upwards and inwards, or, when 

 the valves are closed, are directed into the muscular cavity of the 

 ventral valve ( their upper surfaces slightly grooved )t. Below these 

 first processes, and proceeding from the origin of the thickened 

 cardinal process, another pair of crura are directed inwards, and 

 gently curve towards the first, the medio-longitudinal ridge being 

 continued to the centre of the valve; and on each side, from these 



* In memoriam Professor Amos Eaton, Principal of the Rensselaer School in Troy, from 

 its establishment in 1824, to his death in 1842. 



\ These processes correspond precisely with what are termed the crural processes in Rhyn- 

 chonella, and do not differ materially from the corresponding parts of Rhynchonella psif- 

 tacea, except in their prominence or extension beyond the cavity of the shell as shown in the 

 figures. In the older shells, at least, this process is deeply bifurcate, and grasps the medio- 

 longitudinal septum of the ventral valve, giving additional strength to the hinj e. The ulti- 

 mate extension and form of these processes is still undetermined. Separate valves of the species 

 of this genus are rarely observed, even in positions where the entire shells are common, and 

 where they would have been preserved had they been readily separable like many otherSk 



