38 MULTIVALVES AND BIVALVES. 



Our third division includes all those bivalves in which the 

 pallial line is indented by the impression caused by the insertion of 

 the siphon's retractor muscle. The impression shows that the 

 siphons are long compared to the length of the shell, and the siphons 

 are long because the animal lives buried in the sand and has to 

 thrust these tubes up into the water to be able to breathe. As we 

 have said before, one of these siphons is exhalent, the other inhalent. 

 The inhalent, or branchial siphon, is that near the ventral margin ; 

 the upper one, nearer the beak, is the anal. In the numerous cases 

 in which they end differently, the one that bears the tentacles, or 

 more tentacles than the other, is the branchial siphon. 



This group of genera can be divided into those with one valve 

 larger than another and those with equal valves ; and those with 

 equal valves can be further divided according to the way in which 

 they gape. In this way, we get two groups of three each : 



Shell inequivalve 

 Nearly equilateral. 

 Elongated at posterior end. 

 Truncated at posterior end. 



% 



Shell equivalve 



Gaping at both ends. 

 Gaping at posterior end. 

 Gaping at neither end. 



In the first triad there are only four genera. Lyonsia is nearly 

 equilateral, and any doubt concerning it can be set at rest by a look 

 at the ligament, which is in an internal groove. Pandora is elongated 

 at the hinder end. Thracia and Corbula are both truncated at the 

 hinder end, but Thracia is much larger than the other, and has a 

 crescentic ossicle in its cartilage pit ; while Corbula has no ossicle, 

 and fixes its cartilage in a cavity of its solitary cardinal. In appear- 

 ance the only native species of Corbula is more like a brachiopod 

 than any other bivalve. 



The equivalve shells that gape at both ends can also be divided 

 into three categories. Of the first the sole representative is Mya, 

 which has a broad shell with a broad, short shelf, not unlike the 

 scale of a fish. Of the second the only representative is Lutraria ; 

 its shell is oblong, and very oblique. The third is made up of the 

 Solenidae, and is as follows : 



Shell long and narrow 



Hinge nearly midway, with radiating ribs Ceratisolen. 

 Hinge terminal with one cardinal in the right valve Solen. 

 Hinge sub-terminal with two cardinals in the right valve 

 Solecurtus. 



Ten genera are represented in the group having the shell gaping 

 at the posterior end. Two of them have the cartilage in a triangular 

 cavity under each beak, these being Poromya, which has an oval 

 shell, and Neczra, in which the hinder end is lengthened out into a 

 sort of stalk. Two of them have the shell cut off short at the hinder 

 end, these being Panopaa, which has the pallial line continuous, and 



