OP SOUTHEEN INDIA. 363 



plate somewhat in front and below the apex, while the posterior is very large, 

 elongately ovate, and situated behind the middle of the dorsal margin. 



Locality, — Koloture, in a light brownish conglomeratic rock. 



Formation. — Trichinopoly group, but very close to the boundary of the Arria- 

 loor group, which just in this place is somewhat uncertain, 



XXXYIII. Family,— MTTILIDu^. 



The animals of Mytilibm are distinguished by an elongated, anteriorly nar- 

 rowed or acuminate form ; they have a mantle consisting of several layers, the 

 outer of which is always disunited, secreting on its inner side the horny epidermis, 

 on its outer terminal end the calcareous substance of the shell; the innermost 

 layer is sometimes partially united, and all of them are united dorsally along 

 the ligamental region ; the branchial region and anal opening are variable in extent 

 and form ; there are always two gills and two palpi on each side ; the foot is of 

 variable size, generally small, cylindrical, grooved below, with a large byssal gland 

 at its lower base, the pedal and byssal muscles being strongly developed ; the adduc- 

 tors are always very unequal, the anterior small, marginal near the apex, often very 

 thin ; the posterior conspicuously rounded. 



The shells have an elongately ovate or more or less triangular shape, they are 

 always covered with a horny epidermis, internally more or less nacreous ; the hinge 

 has sometimes a few irregular pliciform teeth, but, as a rule, it is toothless ; the 

 ligament is long, linear, more or less internal ; pallial impression mostly entire, very 

 rarely sinuated ; below the beaks the valves are generally somewhat gaping for the 

 passage of the foot and byssus. 



Most of the species are marine ; a few also occur in brackish water and a few 

 others are fluviatile ; they usually live gregarious and form nests of foreign sub- 

 stances connected by their byssus ; a few have burrowing habits. 



There is not much variation in the form of the shells belonging to the present 

 family, the shape altering between triangularly ovate and elongately ovate, but 

 there are essential differences upon record in the organisation of the animals. 

 According to these I have grouped the various forms in three sub-families, and 

 shall briefly point out the characteristics of each. The fossil genera will be clas- 

 sified according to the most marked relations which the characters of their shells 

 exhibit with recent forms : — 



a, dr:eissenin^. The type of this sub-family is Dreissena polymorpha, Pallas, 

 which has the external mantle margins open and finely ciliated, the internal closed, 

 with one pedal and byssal opening ventrally below the beaks, and two siphonal 

 openings posteriorly, both produced into shorter or longer tubes. The lower or 

 inhalant siphon is larger and ciliated at the margin, the exhalant smaller and 

 smooth ; the palpi are rather small and triangularly elongated, the pedal muscle 

 strong and short ; posterior adductor rounded, large, anterior smaller, attached to 

 the inner side of an umbonal plate. 



4 u 



