MAZATLAN BIVALVES 127 
Modiola caudigera, Lam. 1819, An. s. Vert. vol. vi. p. 27, no. 23.— 
Hanl. Ree. Shells, p. 238.—Phil. Abbild. Conch. vol. 1. 
p. 149, pl. 1, f. 5. 
Mytilus Ropan, Desh. in Lam. loc. cit. (note.) 
Lithodomus caudigerus, Sow.. Gen. f. 4.—Rve. Conch. Syst. 
pl. 99, f. 4. 
Lithodomus lithophagus, Flem. Br.” An. p. ‘414.—Br. Mar. 
Conch. p. 111: (non auct.) 
The 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 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 mar gin. 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 Gastrochena. "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 Gastrochene. 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 
