MAZATLAN BIVALVES 127 



Modiola caudigera, Lam. 1819, An. s. Vert. vol. vi. p. 27, no. 23. 



Hani. Eec. Shells, p. 238. Phil. Ablild. Conch, vol. ii. 



p. 149, pi. 1, f. 5. 



Mytilus Bopan, Desk, in Lam. loc. cit. (note.) 

 Lithodomus caudigerus, Sow. Gen. f. 4. Rve. Conch. Syst. 



pi. 99, f. 4. 

 Litliodomus lithophagus, Flem.' Dr. 'An. p. "414. Br. Mar. 



Conch, p. Ill : (non auct.) 



Tlie Mazatlan specimens vary 'greatly among themselves, 

 being sometimes nearly as narrow as L. attenuatus, at other 

 times approaching in form L. cinnamomeus ; yet they offer no 

 marks by which they can "be separated from the long known 

 W. African species. It begins life, (as may be seen on tracing 

 the lines of growth in the youngest specimen '035 long,) shaped 

 like Unio nuirgaritacea, but more swollen. Soon however the 

 anterior portion is shortened proportionally, while the pos- 

 terior part is prolonged. The umbos from the earliest period 

 are quite flat, and are soon covered by a slight reflexion over 

 them from the anterior margin. The shell is extremely thin, 

 and covered with a glossy chesnut epidermis, turned in over the 

 margin. Soon tubercles appear in regular rows on the epider- 

 mis, which seem to furnish the foundation for the accretion 

 which presently commences. This accretion appears under 

 the microscope as if formed by the agglomeration of particles 

 of the shell into which the creature has bored. It soon covers, 

 more or less, the whole of the epidermis, and is deposited in a 

 thick coating at the posterior end. Here, beginning to appear 

 generally when the shell is about ' 12 long in the form of small 

 terminal knobs, it gradually develops into two shelly spikes, 

 which twist more or less round each other, and are somewhat 

 but never wholly opposite at their bases. These spikes vary 

 greatly in size and shape, sometimes attaining nearly half the 

 length of the shell. They may generally be seen peeping out 

 from the orifice, which is somewhat bilobed, though not so 

 distinctly as in Gastrochsena. When the matrix in which they 

 burrow is not sufficiently solid, they line that part with shelly 

 matter, which occasionally projects as a separate case, as in the 

 British Gastrochamse. This lining is generally found where 

 one burrow crosses another; it was not however universal 

 when crossing the empty part of Imperator. In this shell 

 the creature generally has the instinct to burrow through the 

 thick sutural portion, or else down the axis. In old specimens, 

 the burrow is- often lined with a grayish deposit, apparently 



