263 



NOTES ON THE ANATOMY OF TRACHYC7STIS, BORGASIA, 

 AND ISOMERIA. 



By William Moss and Wilfeed Maek Webb, F.L.S., etc. 



Mead \2>th January, 1899. 



1. Tkachtcystis LYGiEA, Melv. & Pons. 



The genitalia have not hitherto been described, but the external 

 characters of the animal, with the structure of its radula and jaw, led 

 Mr. Pilsbry' to include the present and other species of Trachycystis 

 in a subgenus under Phasis in the family Endodontidge. The authors 

 of the species, on the other hand, have quite recently removed it from 

 Trachycystis and placed it under Zinyis in the Zonitidee.* 



Through the kindness of Mr. Ponsonby the writers have been enabled 

 to examine several specimens from West Africa, and if the result had 

 been made known to him before the publication of his joint paper 

 with Mr. Melvill, it might have induced those authors to retain this 

 species in its original position. The foot of T. lygcBa has a pedal 

 groove, but lacks the tail - like caudal appendage characteristic of 

 Zingis Natalensis and Z. Trottericma. This latter fact and the character 

 of the dentition, in the opinion of the writers, clearly establish the 

 connection of this species with the Endodontidse. In very few genera 

 of Endodontidse is the internal structure known, but Trachycystis lygma 

 agrees with these in having genitalia devoid of accessory organs and the 

 retractor muscle inserted at the apex of the penis. The entrance of 

 the vas deferens into the same part of the penis is another common 

 feature in this family. At first sight T. lygcca would appear to differ 

 in this respect, for at about its middle the penis is joined by the vas 

 deferens, which becomes partially imbedded in the muscular wall of 

 the former until it finally disappears. By transmitted light its lumen is 

 distinguishable throughout the latter part of its course to the apex of 

 the penis. This state of affairs may be what prevails in Amphidoxa, 

 section 8tephanoda, which Pilsbry describes as having a vas deferens 

 which " passes gradually into the penis " ; but the figure given in the 

 Manual of Conchology^ is not sufficiently detailed to enable an 

 opinion to be definitely formed. Furthermore, in T. lyg(Ba the duct 

 of the spermatheca is long, as it is in Pyramidula} 



' Man. Conch., ser. ii, vol. ix, p. 37. 



* " A Contribution towards a Check-list of the Non-Marine MoUuacan Fauna of 



South Africa" : Proc. Malac. Soc, vol. iii, pp. 173 and 171. 

 3 T.c, p. 40, pi. i, lig. 16. 



* Man. Conch., t.c, p. 42, pi. xi, e.g. Fattila alternata, Say, fig. 20. 



