ALARIA. 109 



Aporrhais, and, if we except certain obscure forms with no wing attached, the 

 monodactyls seem to have been the earliest Aporrhaids. Even the didactyl Alarice 

 have the wing but scantily palmate, and not embracing. In the Great Oolite, 

 however, there are forms such as " Pterocera " atractoides, Desk, where the wing 

 becomes more complex and embracing ; and here the resemblance to Aporrhais 

 becomes effective. Hence Piette places this species under Aporrhais. When we 

 come to the malacology of the subject, attempts at comparison resolve themselves 

 mainly into conjecture, as far as the Jurassic Aporrhaids are concerned. These 

 may have been the ancestors of part of the Strombidae as well as of the modern 

 Aporrha'ideB, and probably differed from both to a certain extent. Piette divided 

 the Jurassic Alarice into five sections, viz. Varicifers, the Monodactyls, the 

 Adactyls, the Longicaudes, and the Hamicaudes. To simplify matters these might 

 be placed under two grand divisions. We do not, in this country, appear to have 

 the section " Varicifer " developed, as far as I can understand it ; the Monodactyls 

 proper are numerous and important, whilst the Adactyls may owe their wingless 

 appearance to various causes. All these I propose to place under the first grand 

 division, assuming that they either have, or might have, no more than one digita- 

 tion when there is no evidence of a second. Our first grand division, then, is 

 constituted by the Monodactyls. 



The Longicaudes, which are almost coextensive with the myunis-gromp, and the 

 Hamicaudes, which are almost coextensive with the trifida-growp, make up the 

 second grand division. The shells of both these groups, when mature, develop two 

 digitations on the wing. These are Didactyls. There will still remain one or two 

 forms somewhat difficult to place. 



It is probably true that more species are made out of these fossils than would 

 be the case if all the specimens could be obtained in a reasonably perfect condition, 

 like the shells, for instance, of existing species of Aporrhais. But if we were to 

 wait until none but perfect specimens of Alaria were accepted, the Jurassic Apor- 

 rhaidse might as well be left alone. Owing to the number of processes which many 

 of these curious shells possessed, their preservation is rarely complete, and it thus 

 happens that what ought to be recognised as the same species presents such a 

 different aspect under the various conditions under which it is found as to make its 

 recognition very difficult. To avoid error altogether under the circumstances is 

 almost impossible. Too often we have to choose between the Scylla of a doubtful 

 identification and the Oharybdis of " species-making." 



Thus the first forms described and figured in PI. IV are wingless, and mostly 

 without the canal-sheath. It becomes necessary to distinguish these in some way, 

 yet such "species" are little more than tentative. The bulk of the species are 

 described with some attempt at grouping. 



