98 



always founded on the shell, but sometimes, where the shells of two species are 

 undistinguishable from each other, their tubes or pallets may afford considera- 

 ble differences. The pallets alone as will be seen indicate two distinct genera, 

 where the valves do not differ. Hence it is necessary, in many cases, for a 

 certain determination of the species, that the valves, tube, and pallets shall 

 each be examined, and it is needless to expatiate on the confusion which would 

 arise from the accidental commingling of the tubes or pallets of one species 

 with the valves of another; and this confusion is more apt to occur when, as 

 is not unfreqnently the case, several species are found inhabiting the same 

 piece of wood, and being broken in their extraction, the pallets and valves fall 

 out indiscriminately intermingled. 



Another difficulty in the study of the Teredidse is the great variation of the 

 individuals in size, proportions, and markings, making an accurate diagnosis a 

 simple impossibility, and compelling us to rely on a general accordance with de- 

 scriptions in the most material points. Mr. Hanley remarks that "there is one 

 fact with regard to the shipworms, which has rendered their investigation 

 peculiarly laborious, namely, that no reliance can be placed upon the relative 

 proportions of their several parts for specific definition. If we take at random 

 about fifty valves of Norvagica, for instance, we shall find that in some the 

 oblique decussated strise occupy twice the space of the succeeding strip, in 

 others this is reversed, in many these are both contracted, and a large pos- 

 terior smooth area is exhibited ; in others again almost the entire surface is 

 occupied by the two former, to the great diminution of the hinder portion. 

 Hence it is absolutely necessary to examine very numerous examples in order 

 to elicit the real and permanent specific characters, and the valves alone are 

 rarely adequate for the determination of the species." 



Dr. Gray proposed, in 1851, to consider the Teredines a subfamily of Phola- 

 v\dm, but Mr. P. P. Carpenter has separated them under the name of Tere- 

 did.e, with great propriety, as they undoubtedly exhibit sufficient differences 

 from the Pholades, and from all other Mollusca, to be entitled to the position of 

 a family. 



I have already given (in Proc. A. N. S., April, 1862) a sketch of the division 

 of Teredid;e into three subfamilies, which it will be necessary to reproduce 

 here : — 



Family TEREDIDJE. 



Animal elongate, subcylindrical, siphons united nearly to the end, their ex- 

 tremities armed with two shelly styles ; (Pallets.) foot long and narrow, protruded 

 through the united mantle lobes, which are thickened in front. Gills long; 

 mouth with palpi. Shell, when present, globular, tripartite, included with the 

 animal in a more or less cylindrical testaceous tube, the siphonal end of which 

 is divided into two by a longitudinal partition. 



Subfamily 1 . Teredinje. Valves present, free, contained in the tube, which 

 is irregularly cylindrical, sometimes much contorted. Perforating timber. 



Subfamily 2. Teredininje. Valves with an accessory anterior dorsal plate, 

 their margins prolonged into a shelly tube when adult.* Tube frequently con- 

 camerated ; siphonal extremity often truncate, and the opening contracted by 

 a six-lobed internal margin (fossil). 



* Dr. Gray supposes the fossil genus Teredina to be more closely connected with Pholamm: than 

 with TeredidjE, from the fact that the shell has an accessory dorsal plate, and is external to the 

 tube. It must be confessed that the genus is curiously related to all three families ; the external 

 position of the valves, and the lobed end of the tube, exhibiting an approach to the Gastroch^i- 

 Nn>-E. I have concluded to place it for the present in Teredid.«, in a position where it may indicate 

 a transition from the free and perfect valves of Teredo, through its less important valves event- 

 ually becoming merely a portion of the tube, to the Kuphcs, where the valves are entirely want- 

 ing, or are replaced by the cleft shelly plate which closes the lower end. 



