802 



PROP. G. C. BOURNE ON THE 



projection at the peristome. Aperture semilunar, very oblique, 

 the outer margin thickened and expanded. Basal callus not 

 very thick, hardly differing in colour from the rest of the shell, 

 of rather small extent, its limits clearly defined above, as well as 

 below. The whole shell deej^er in proportion to its breadth than 

 in A . andamanica and the sjoire more prominent. 



Operculum yellowish white in colour, the calcareous plate 

 rather thin, the sigmoid curve pronounced, the upper angle 

 produced. 



Radula with small diamond-shaped median tooth ; the first 

 admedian tooth more than twice as long as broad, the anterior 

 edge incurved and bearing four denticulations ; second and third 

 admedian teeth each with four denticulations ; the lateral teeth 

 normal, the stalk rather long, the articular excavation deep, the 

 aliform process bearing seven round denticulations. 



Closely as the shell of this species resembles that of A. anda- 

 mauica, a glance at the drawings of the radulpe of the two species 

 (figs. 63 & 65) shows that they are distinct. 



I must express my obligations to Miss Margaret Poole, both 

 for helping me in the determination of the different species of 

 Aphanoconia and for making the drawings of shells and radulse 

 for figs. 62 to 68. 



As I have discussed and offered an explanation of the geo- 

 graphical distribution of the Helinicidte without either adopting 

 or ci-iticizing the theories advanced by Dr. Simroth (7 and 8) on 

 this subject, I must, in conclusion, make some reply to the friendly 

 criticisms that he has published on my paper on the Neritidfe. 

 I do not propose, in this place, to discuss the physiological 

 interpretation that he has given of the different arrangements of 

 the female ducts in the Neritidfe. For one thing, I have 

 obtained some new material and hope soon to publish further 

 observatioiis throwing fresh light upon the problems to which 

 he refers : for another, I am inclined to accept much of what he 

 writes on that part of the subject. 



. But with regard to the ancestry of the Neritidpe, and with them 

 the Helicinidfe, which Di'. Simroth would derive from a pulmonate 

 stock, and with regard to the homologies that he wishes to establish 

 between the generative ducts of Neritidfe and Pulmonata, I am 

 unable to accept any of his conclusions. To do so would be 

 to throw the whole fabric of morphological reasoning to the 

 ground. Dr. Simroth's views on homologies are largely influenced 

 by a theory of secular changes in the sea-level produced by a 

 swinging or " pendulating " movement of the earth about an axis 

 which corresponds with the longest diameter of the earth and has 

 its poles in Sumatra and Ecuador. It is not my present intention 

 to discuss the difficult astronomical and geological problems in- 

 volved in the " Pendulation theory," and, indeed, I am sure that T 

 am incompetent to discuss them. The theory may be well founded 

 or it may not : I do not offer an opinion ; but be it right or 



