MYTILARCA. 117 



I have been quite unable to find any genus in which there appears to be the 

 slightest ground for placing the following species. 



1. DlGONIOMYA ELEGANS. Plate XIII, fig. 13. 



Description. — Left valve large, oblique, lozenge-shaped, very transverse, 

 convex. Umbo small, very sharp, oblique, incurved, and prominent, situate very 

 anteriorly, being only about one-tenth of the length of the shell from the anterior 

 side, and excavated along its back by a concave depression which is continued to 

 the middle of the hind margin. Hinge-line about two-thirds the length of shell, 

 straight. Anterior margin very narrow, subangular. Inferior margin very long 

 and slightly sigmoid, being a little concave towards the rear. Postero-inferior 

 corner very much produced, narrow, and so sharply rounded as to be almost 

 subangular. Posterior side very oblique, long, nearly straight. Contour of surface 

 very convex vertically ; the front and back of the umbo being continued in low 

 rounded prominences to the antero-inferior and postero-inferior corners, between 

 and behind which the shell is slightly concave. Surface apparently covered with a 

 few irregular growth-lines. 



Size. — Length 60 mm., height 20 mm., depth of one valve 10 mm. 



Localities. — In the Musenm of Practical Geology is a specimen from Croyde. 



Remarks. — I have been unable to refer this shell to any known genus. While 

 remarkable in form, its state of preservation is not such as to give full information 

 as to its nature. 



III. Family — Aviculid2e, d'Orbigny, 1843. 



I. Sub-family — Ambonychin^, Miller. 



1. Genus — Mytilaroa, Hall, 1870. 



The following species is provisionally placed under this genus solely on account 

 of its general shape, and probably may have to be transferred elsewhere should 

 clearer specimens be found. 



1. Mytilaroa? modioloides, n.sp. Plate XIII, fig. 8. 



Description. — Left valve of moderate size, very oblique, subtriangular, convex. 

 Anterior side very narrow and convex. Umbo apparently situated at or close to 



