92 CRETACEOUS PELECYPODA 



There are no fossil species known of either of the two genera, but among the 

 various forms of the Jurassic Myce there are several which externally, greatlv 

 resemble Glauconomyce ; the knowledge of the hinge-teeth is essential for their 

 correct generic determination, the external shape of the shells being in such 

 cases entirely unreliable. 



VIII. Family— SOLJENIDJE. 



The animals included in the present family are distinguished by their great 

 length in proportion to their height. The mantle is united, except in front and 

 behind, for the respective passages of the foot and the siphons. On account of this 

 great length and cylindrical form of the body the Solenid^ are usually classified 

 next to the Frolabid^, although on closer inspection the relations are by no means 

 so clearly understood. In the Frolabi-dm we have always a small pointed foot, 

 and the posterior part of the body in which lie the gills greatly elongated, none of 

 which are found in the Soljenid^. With regard to the last organ, the gills as well 

 as respecting the habits of the animals, — (and the same applies to the form and 

 character of the shell, as already noticed) — their closest allies unquestionably 

 appear to be the Glauconoiiyijdm and Saxicavid^. On the other hand, the 

 SoLENiDM, all have a long, very strong foot, which often occupies half the length, 

 or more, of the entire body ; the siphons become sometimes perfectly divided from 

 their bases to their terminations, as, for instance, in Novaculina, and thus very 

 much recall some of the elongated forms of the Tellinid^. In fact some of the 

 species of Solecurtus and Siliqua indicate through the form of their shell and its 

 dentition on the hinge a direct passage from the present to the next family. 

 Eor these reasons I prefer rather to follow in the present arrangement of the 

 SoLENiBJE the one adopted by Forbes and Hani ey in their admirable ''History 

 of British MoUusca," than that of more recent writers. 



The foot of the Solenid^ consists of a strong muscular tissue, and is, ac- 

 cording to the observations of Clark, provided in almost its entire length with an 

 aquiferous canal. "When protruded out of the shell it is clavate in front, but 

 when employed in burrowing its termination can extend into a lingui-form point, 

 which by being twisted from one side to the other forms hollows in sand, in which 

 the animal can burrow with the greatest rapidity. Deshayes has observed that 

 the animal possesses besides the power of progressive motion. It fills its body with 

 water, and by ejecting the same forcibly through the siphons darts from one place to 

 the other, till it can find a convenient one to burrow in. I have had opportunity of 

 observing this kind of motion repeatedly in Fharella^ Novaculina, and others, but the 

 greater part of the water was during this kind of progressive motion ejected through 

 the opening below the siphons, rather than through the exhaling siphon itself. The 

 mouth is surrounded as usual by triangular palpi, generally of no considerable size. 

 The siphons are either united nearly the whole of their length, as in the soLENiNjEy 

 or more or less separated, as in the trarellinm. The other characters of the animals 



