168 Annals of the South African Museum. 



The measurements agreeing most nearly with those given in 

 Gray's description, of a specimen in his Type set, are : Diam. 

 maj. 32-6, min. 24'8 ; alt. max. 15-5; apert. 13'7 x 1V7 mm. 



Animal only known from a single, slightly immature specimen, 

 from " Herero and Namaland " ! ? examined by Simroth and Pilsbry. 

 According to their descriptions and figures, the possession of the 

 following characters distinguishes the typical form of I), alexandri 

 from the other members of the genus. 



Pedal gland lying free in the body-cavity. Jaw * " entirely 

 smooth," with little or no median projection. Eadula t with teeth 

 resembling those of the var. rotundata, but broader in front, and 

 the centrals and laterals with longer basal plates. Reproductive 

 system J : free oviduct not swollen, vagina long, vas deferens not 

 attached to the side of the penis, but free throughout ; penis rather 

 long, becoming thicker and bending abruptly towards the hind end, 

 extending 1^ mm. beyond its junction with the vas deferens, to form 

 a short terminal caecum or flagellum, on the apex of which the penial 

 retractor is inserted. 



Hab. GREAT NAMALAND. Neighbourhood of the Great Fish 

 Eiver. LITTLE NAMALAND. Neighbourhood of the Orange River. 



Type set in British Museum. 



Judging from the descriptions of Simroth and Pilsbry the typical 

 form of D. alexandri differs widely from the other known members 

 of the genus in possessing a smooth jaw and a terminal caecum on 

 the penis ; for in all the remaining forms that have been examined 

 the jaw is transversely striated, and the penis scarcely extends at all 

 beyond its union with the vas deferens, as will be seen from the 

 figures. It must be remembered, however, that the observations of 

 both these authors were made on the same specimen, which was 

 stated to be badly preserved and slightly immature; and it is 

 therefore to be hoped that further material will be collected in 

 order that a new examination of the anatomy may be made. 



Many varieties have been attributed to this species. The typical 

 form is chiefly known from the neighbourhood of the Lower Orange 

 River ; I have seen one beautiful example of dark reddish-corneous 

 hue, and it is possible that when further good species have been 

 collected this coloration will be found to prevail in the compara- 

 tively smooth typical form, and to supply additional ground for its 

 specific separation from varieties such as rotundata. 



* Man. of Conch., 1895, ix. PI. LX, f. 3. 

 f Ibid. PI. LX, f. 6. 



I Ibid., frontispiece, f. 3, and Ber. Senckenb. Naturf. Ges. Frankfurt, 1894, 

 p. 94, text-figs. 



