INTKODUCTIOX. XI 



It ■will be noticed (p. 7) that I have retained Adaon in its usual 

 place, amongst the OpismoBRANcniAXA ; but it must be confessed 

 that this is done with some misgiving. It is generally concluded,' 

 judging from the structure of its shell and its persistent operculum, 

 that that genus should be placed amongst the lower Opisthobranchs; 

 but it should be observed that the earlier portion of the protoconch 

 is distinctly sinistral, and has all the essential characteristics of 

 the Pyramidellid^. On that ground alone one might feel inclined 

 to classify Aetceon, and possibly some of its associated genera, 

 amongst the Peosobranchiata. We should be strongly fortified 

 in so doing by the important discovery by Bouvier,- in a careful 

 anatomical investigation, that Aetceon has a twist in its visceral 

 nerve-loop, and is, consequently, streptoneurous. In view of 

 what is known of the general organization of the genus, even 

 in spite of the evidence afforded by the protoconch and the study 

 of the nervous system, we are hardly warranted, however, in 

 removing it from the Opisthobranchs ; and for the present it will 

 be safe to conclude that we are here dealing with inherited 

 streptoneury.' 



The value of the elevation of the spire in the classification of 

 the C0NID.E is discounted by what we find in the ontogeny of 

 Conus cuspidatus (p. 32), where the amount of elevation is shown 

 to be merely a peculiarity of the individual. 



The impossibility of satisfactorily diiferentiating Stcrcula from 

 Pleurotoma when dealing with fossil specimens, is insisted upon 

 (p. 37) ; and, incidentally, the position of the sinus as a prime 

 element in the classification of the Pleurotomid^ is criticized. 

 Although the majority of protoconchs in species of the Gastero- 

 poda appear to be smooth and without sculpture, many exceptions 

 are noted. In particular, Pleurotoma icangamiieiisis (p. 46) is 

 remarkable in that respect. The initial turn of the protoconch 

 is smooth and obsoletely striated, and this is followed by an obtuse 

 angulation, which rapidly develops into a bold, acute, median spiral 

 carina, with accompanying lineation. See also Columharium acan- 

 thoslephes (p. 52), Telcoclulus gracillimuin (p. 65), Harpa ahhreviata 



' Howes, Proc. Make. Soc. London, vol. ii. 1896, p. 68. 

 * Bull. Soc. Pliilom. Taris, t. v. 1893, p. 64. 



