272 



Recent MoUusca.' These naturalists support their views by a fact 

 mentioned first by Blainville, and confirmed by Souleyet, that the 

 animal of T. maculata has the tentacles very thin and short, carrying 

 the eyes at their summits. Messrs. Quoy and Gaimard on the con- 

 trary prove that in T. dimidiata, referred notwithstanding to the 

 genus Acus of Humphrey by the Messrs. Adams, the eyes are placed 

 at the middle of the tentacles. 



These are the very words of these naturalists ('Zoology of the Voyage 

 of the Astrolabe,' vol. ii. p. 462) : — " The animal has a tolerably large 

 head ; the distant tentacles are excessively thin and short, and one 

 can hardly perceive the eyes about the middle of their length.'''' 



On the subject of the Terehra, the same naturalists add some 

 interesting details. " This species {Terehra subulatd) is figured," 

 they say, "at page 465, to show that the animal does not differ from 

 the preceding {T. dimidiata) ; its head is prolonged into the form of 

 a little muzzle, which disappears when the siphon expands and is 

 put out ; the tentacles are bigger and less distinct. We believe that 

 in that one which we saw the eyes were placed altogether at their 

 extremities ; this arose probably from the contraction of the points 

 which surmount them." 



Hence, according to the facts established by Blainville, Souleyet, 

 and Quoy and Gaimard, there would be in the group of Acus, as re- 

 employed by the Messrs. Adams, some species having the eyes at the 

 tip of the tentacles, and others bearing them at the middle of those 

 organs. It also follows that in the Terehra proper of the Messrs. 

 Adams, to which T. subidata belongs, the eyes appeared situated at 

 the extremity of the tentacles, if we admit the doubt expressed by 

 M. Quoy on the subject of the contraction of the upper ends of the 

 tentacles. 



What has just been said will tend to weaken much this division 

 into two genera of the species belonging to the old genus Terehra of 

 Lamarck. Before we admit the genus Acus, we think it more pru- 

 dent and wise to await the result of further observations. This 

 seems the more necessary, as in the shells we do not notice any con- 

 stant character by the help of which we could distinctly and easily 

 separate the genera. There are found a great number of links 

 between the different forms of the aperture of the columella, of the 

 notch, and of the short terminal canal. The external form and the 

 diversity of ornamentation, are repeated from one group to the other. 

 Also, in examining the characters given by the Messrs. Adams, we 

 find that the most important, and that to which these authors attri- 

 bute the most vahie, is borrowed from the animal ; that is to say, 

 the position of the eyes on the tentacles. 



It might, perhaps, be interesting to put forth the various opinions 

 of naturalists upon the relative position to be assigned to the genus 

 which occupies us at present, in a natural arrangement of molluscous 

 animals ; but, to show with certainty the opinions successively ad- 

 vanced, it would be necessary to enumerate in detail all the facts 

 already known respecting the organization of the animal, to deduce 

 from these facts the whole of the differences and resemblances with 



