187 



the area anterior to the umbonal ridge is not so precipitous,, 

 and the ridge is not so elevated as in the recent species. The- 

 radial threads are not so elevated, and are separated by nar- 

 rower furrows ; the concentric ornament is, moreover, finer. 

 The anterior adductor boss is peculiar. 



When describing Septifer fenestratus I had only one speci- 

 men of Mytilus Hamiltonensis before me, which I then re- 

 garded as an adult shell of that species; the observations 

 published thereupon should be erased from the diagnosis of 

 Septifer fenestratus. These refer to the filling up of the um- 

 bonal cavity and the dimensions of the shell. 



Mytilus linguatulus, spec. nov. Plate six., fig. 3. 



Narrowly oval, rather compressed ; umbones uncinate ; an- 

 terior margin nearly straight, the dorsal and posterior margins 

 forming an interrupted curve ; anterior side very narrow, des- 

 cending abruptly from the low umbonal ridge. The surface is 

 ornamented with elevated rounded radial threads, rather nar- 

 rower than the flat furrows, and strong folds of growth and 

 fine concentric wrinkles. The margin of the valves is crenulated 

 all round. There is an elongated tooth under the hinge ; and 

 the anterior adductor is raised on a prominent boss. The- 

 byssal insinuation is inconspicuous. 



Dimensions. — Umbo-ventral diameter, 48 ; greatest transverse 

 diameter at right angles, 19; sectional diameter, 16 milli- 

 metres. 



Locality. — Upper beds at Muddy Creek (J. Dennant!) 



This species is very like M. rostratus, Dunker, but wants the 

 angulation of the dorsal margin ; the beaks are more pro- 

 duced, slightly excavated, not swollen, beneath and in front of 

 the umbo ; it has one long tooth in place of two or three short 

 ones, and the anterior adductor boss is peculiar. 



Mytilus deperditus, spec. nov. Plate xix., fig. 1. 



Oblong ; test thick, especially about the umbo, which is ob- 

 tuse and swollen. Surface ornamented with strong folds and 

 striae of growth. Margin of valve plain ; hinge with one- 

 large tooth. 



Dimensions. — Umbo - ventral diameter, 80 ; transverse- 

 diameter towards the extremity of the hinge line, 35 ; sectional 

 diameter of one valve, 16 ; length of hinge line, 60 milli- 

 metres. 



Locality. — Upper beds at Muddy Creek {J. Dennant I) 



M. deperditus resembles M. latus, Chemnitz (non Lamarck),, 

 but presents the following differential characters : — Narrower, 

 the anterior side steeper, straighter, and longer hinge lines, 

 stronger growth-folds, and convex, not concave, under the- 

 umbo. 



