MYALINA SUBLAMELLOSA. 121 



text lie states that only two specimens had been obtained. This species differs 

 from most others in this genus in possessing a shell less acutely bent in front ; 

 and therefore it has not a flat anterior surface. The ridge is replaced by a more 

 general gibbosity. De Koninck says, " Une forte gibbosite, ayant son origine au 

 sommet des crochets, domine les valves, se rend en courbe vers l'extremite in- 

 ferieur du bord anterieur." In his remarks he notes the resemblance between his 

 species and Myalina recurvirostris of Meek and Worthen (' Geological Survey of 

 Illinois,' vol. ii, ' Palaeontology,' p. 344, pi. xxvi, figs. 9 a — c), to which there is a 

 very close resemblance; but this species seems to have had a great development of 

 the umbones forwards, for they are shown to project about one-sixteenth of an inch 

 in front of the rest of the shell ; and the beak of the left valve is said to be " twisted," 

 so as to make nearly one entire turn at the extreme point, which is directed obliquely 

 forwards. A curious error has been made in the drawing. The striae of the hinge- 

 plate are depicted as more numerous, and the hinge-plate thicker and more ex- 

 panded, behind than in front — a condition which would be impossible from the 

 method of shell-growth from the umbones backwards. 



The difference in shape between the cast and the shell is to be noted. This is 

 due to the much-thickened anterior portion of the shell, and the non-correspondence 

 in position and size of the umbones in the shell and the cast. 



This species differs from M. lamellosa, of which it may after all only be a 

 variety, simply in shape, being less oblique and having a longer hinge-line. Both 

 forms occur together at Hill Bolton and Congleton Edge. 



Myalina sublamellosa, B. Etheridge, jun., 1877. Plate IV, fig. 15 ; Plate V, 



figs. 4 — 8. 



Myalina sttblamellosa, R. Etheridge, jun., 1878. Quart. Journ. Geol. Soc, 



vol. xxxiv, p. 14, pi. i, fig. 15; 

 pi. ii, figs. 16, 17. 

 — — JR. Etheridge, sen., 1885. Brit. Toss., part i, Palaooz., 



p. 286. 



Specific Characters. — Shell flattened, trigonal, the anterior side obsolete. The 

 anterior margin truncated, with a slightly sigmoidal margin, descending from the 

 beak at rather less than a right angle. The ventral (inferior) margin is convex, 

 passing upwards before and behind with a gradual curve into the anterior and 

 posterior margins. The posterior part of the shell is flattened, with its margin 

 slightly convex or subtruncate. The hinge-line is straight, equal in length to a 

 little less than the greatest antero-posterior diameter, parallel to it, making behind 



16 



