Dr. J. E. Gray on the Structure and Position o/Terediua. 89 



by some observers, as mentioned by Mr. G. B. Sowerbv, for 

 opercula or palettes. 



3. The gape in the front of the valves is filled up, as is the 

 case with the adult specimens of Martesia; but here it is filled 

 up with a single shelly plate, leaving only a small perpendicular 

 slit in the middle of the well-marked medial line, while in Mar- 

 tesia the plates filling up the gape in each valve are always sepa- 

 rate. This plate, as in Martesia, is formed by the gradual addi- 

 tion of shelly matter to the front margin of the gape of the 

 valves, until it reaches the middle, where the two plates are 

 united into one, leaving the central slit. The development of 

 this plate is shown by the concentric lines of growth which are 

 to be observed on its surface in well-preserved specimens. 



4. The hinder part of the cardinal and ventral margins of 

 Teredina is destitute of any additional shelly plates, which are 

 generally developed in the species of the genus Martesia. 



I think, after these comparisons, that we may conclude that 

 Teredina is a genus of Pholadina allied to Martesia, but sufii- 

 ciently distinct from it to be retained as a separate genus, as I 

 suggested in my paper "on the Arrangement of Pholadida into 

 Natural Groups," Ann. & Mag. Xat. Hist. viii. 1851, p. 384. 



I believe that at present only a single species of the genus is 

 known, which varies much in size according to the position 

 it holds in the colony, some obtaining 

 more and others less nourishment, from 

 local causes. 



They present so much variety in the 

 form and state of development of the 

 additional dorsal valve or bouclier, that 

 if they were not all collected together 

 from the same colony on the same piece 

 of rock, one might be induced to con- 

 sider them as belonging to two or more - . i 

 distinct species. \ ! 



In some specimens the dorsal shield 

 is small, placed entirely in front of the 

 umbo, looking like a flattened bag, nar- 

 row in front, rather wider and more con- 

 vex behind, and slightly contracted on 

 the sides (fig. 2). In this state it is 

 figured by M. Deshayes (Coq. Fossiles 

 de Paris, tab. 1. f. 24). 



In others it is large, convex, sub- 

 quadrangular, covering the umbo and 

 the upper central part of the base of 



Teredina personata. 

 Showing how the siphons 

 bulge over tiie valves. 1. In 

 usual state, without the dor- 

 sal i)late; 2, with a small, 

 3, with a large or perfectly 



the siphon, which is prominent behind developeil dorsal plate 



