MAZATLAN BIVALVES- 



127 



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



Hani. Bee. Shells, p. 238. — Phil. Abhild. Conch, vol. ii. 



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

 Mytiliis Ropan, Desh. in Lam. loc. cit. (note.) 

 Lithodomus caudigerus. Sow.. Gen. f. 4. — Sve." Conch. Sysf. 



pi. 99, f. 4. 

 Lithodomus lithophagus, Flem.'Br.'An. p. '414. — Br. Mar. 



Conch, p. Ill : (nou auct.) 



The M azatlan specimens vary 'gi'eatly among themselves, 

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

 times ai^proaching 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 Uuio margaritacea, 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 tliin, 

 and covered with a glossy chesnut eijidermis, 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 appeai-s under 

 the microscope as if formed by the agglomeration of particles 

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

 more or less, the whole of the epidermis, and is dexiosited 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 Gastrocha;na. AVhen the matrix in ^hich they 

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

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

 British Grastrocha?na3. This lining is generally found where 

 one burrow crosses another; it was not however imiversal 

 when crossing the em])ty part of Imperator. In this shell 

 the creatm-e generally has the instinct to burrow tlu-ough the 

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

 the burrow is often lined with a grayish deposit, apparently 



