88 Dr. J. E. Gray on the Structure and Position of Terediua. 



was soft and flexible when the animal was living. There is little 

 doubt that in the young state the large rhombic gape between 

 the front of the valves is open, as in Teredo, Zvjma, and the 

 Youna: of Martesia, though I have never seen a specimen show- 

 ing this state of the animal, but always with this part closed, as 

 in the adult example of the latter and several other genera ot 



PholadidtE. . i ir j 



Among the recent genera of Pholadina it is most nearly allied 

 to Martesia, but it differs from that genus m several important 

 particulars, which may be thus stated :— ^ , ■ r, 



1. The valves are small, compared with the size of the siphons, 

 and the gape is larger and more rhombic. , . , , , 



2. The siphons are much larger, compared with the valves 

 and shell, and swollen beyond them ; in the fossil state they 

 are covered with a hard calcareous coat, or rather coats, tor the 

 part which is described as the tube is formed of scvcra. concen- 

 tric calcareous layers. These siphons, though in the tossil state 

 they are hard and shelly, have all the appearance of havmg 

 been soft and flexible in the living animal, like the siphons ot 

 ZirUr, crispato, which they greatly resemble in appearance; tor 

 they are bent in various directions, and are swollen bepnd the 

 edKC of the valves behind, so as to have all the bag-like ap- 

 pearance which the fleshy flexible siphons of these bivalves 



present. , , 



The surface of the stony siphons of the best-preserved speci- 

 mens of Teredina presents the minutely wrinkled surface which 

 is to be observed on the more or less coriaceous coat sinnlar to 

 the periostraca of the valves which envelope the fleshy siphons 

 of Pholades, Mya>, and others which have large, constantly ex- 

 posed siphons. ^ , . , „ , . 



It is to be observed, that the shelly cast of the siphons of this 

 genus generally presents two very different ai)pearances : the 

 lower portion nearest the valves is usually hollow, and formed oi 

 several thin, concentric, shelly lamiufc, as is easily seen when it is 

 broken across. The upper portion is only provided with a thm, 

 easily deciduous, shelly coat, tilled withm with a brown solid nu- 

 cleus, pierced in the centre with two more or less distinct tubes. 

 This portion appears to represent the part of the two siphons 

 which is united together; as it is easily separated irom the lower 

 portion, many specimens contained in collections are desti- 

 tutc of this part, and, from T.amarck and Dc lilaniville s de- 

 scriptions, I should suspect that it was wanting m the specimens 

 they described. The two portions are well Hgured in Sowerby s 

 'Genera of Shells/ Hg. 1, and the lower portion, showing the 

 laminated appearance of this part, in flg. 3 of the same plate. 

 It is probable that the upper portion may have been mistaken 



