64 DAVIS : BIOXOMICAI, CONSIDERATIONS IN GASTROPOD EVOLUTION. 



been more completely removed from the proximity of the left ctenidium and 

 its incurrent stream. With this shift of apertures to the right, a lengthening 

 of the rectum, &c., has evohed j^ari jmssji, so that the anus, &c., tend to move 

 forwards on the right side. In this improved arrangement the slit is no 

 longer necessary and— as it must have always been a week point in the 

 structure of the shell — has disapperred. It has also been lost in the Doco- 

 glossa, where a slit would undoubtedly militate against firm attachment, and 

 its transfer in the Fissurellidae to the posterior part of the branchial cavity 

 has not only minimised the reduction in the strength of the shell due to its 

 presence, but also promoted etificient flushing in a special way. For it is just 

 above the anus, so that the excurrent stream is as short as possible and does 

 not take, as in Haliotis, &;c., a direction contrary to the incurrent streams 

 with resulting interference with the flow of both it and them. It may, how- 

 ever, be noted that in Haliotis a part of the excurrent stream makes its exit 

 form each of the series of holes into which the slit is converted — just as it 

 would do from the successive parts of a continuous slit — and the conversion 

 of the slit into a number of holes also prevents the shell from being too much 

 weakened. 



In the foregoing article my aim has simply lieen to make a short pre- 

 liminary statement of the results to which researches on the habits of Mollusca 

 and the connected structural adaptations have led me. 



I wish to gratefully acknowledge much kind help, and many valuable 

 suggestions from my friend and colleague Dr. H. J. Fleure, who has been 

 closely associated with me for some years in researches upon the structure 

 and habits of Molluscs, especially of Gastropods, and in speculations on their 

 phylogeny. Further articles, by one or both of us, will enter into a fuller 

 discussion of the subject matter of this excursus, giving further details, and 

 reviewing the relevant literature. 



CURRENT LITERATURE. 



Pilsbry, H. A.— Manual of Conchology, ser. ii, vol. xvii (pt. 68), pp. 209—232 xi-xviii, 

 pis. _j^ 65. Philadelphia : Academy of Natural Sciences. 



With the issue of part 68 Dr. Pilsbry completes another volume of this invaluable 

 monograph, and commences the seventeenth volume which is to be devoted to the 

 .\frican A chalin'ulac. 



In an interesting introduction he points out thaf these molluscs differ from the 

 Biiliiuiiliiiac by the long kidney, from the Acavidac by the diverse venation of the lung 

 and the d fferent dentition, and from the Helicidac chiefly by the narrow central tooth 

 of the ladula and the structure of the shell. So far no slug-like Achatinidac are known, 

 but Dr. Pilsbry belives such to exist. 



The African species are classiticd in three groups or sub families, vi/., Achaliuiiiac, 

 Ste)iogyriiiae, and Codiaxiiiae. 



The first sub-family comprises about a dozen genera. With three exceptions 

 all the forms conform closely to a common type. Very brief particulars are then 

 given of the generative and other organs in the different genera, together with a key 

 based on shell characters. Of the anatomy of the second sub-family but little is 

 known. 



