KErOET ON THE GASTEROPODA. 579 



3. Cithna, A. Adams, 1863. 



This genus was proposed by A. Adams (Proc. Zool. Soc. Lond., 1863, p. 113), as a subgenus of 

 Conradia, regarded by him as one of his family Fossaridce. Its characteristics do not stand out very 

 clearly, for he defines the subgenus as " umbilico carina semilunari extus instructo," but he adds 

 explanatorily that " it resembles Conradia, without any ridges or keels on the whorls." Then he 

 describes Cithna globosa as " umbilico extus valde carinato," and regarding Cithna spirata besides its 

 umbilicus " carina conspicua circumcincto," he says that it has whorls " superne angulatis ultimo ad 

 peripheriam carina transversa instructo," so that there is here a species with three keels, although as 

 a Cithna it should have had none. 



Dr Gwyn Jeffreys ("Lightning" and "Porcupine" Moll., Proc. Zool. Soc. Lond., 1883, p. 110) 

 has removed this subgenus from its connection with Conradia, and put it as a genus (unfortunately 

 without fresh definition) next to Lacuna, from which he says " it differs in being destitute of an 

 epidermis, and in having, instead of a flattened and channelled pillar, an obliquely curved umbilical 

 chink, which ends in a small but deep perforation, and is enclosed by a more or less sharp and 

 distinct ridge. The tentacles of the animal are ciliated, as in Trochus and Eissoa, which is not the 

 case in Lacuna." 



With few exceptions, and these of a pecuhar and Hmited character, there are probably no shells 

 wholly destitute of epidermis, however caducous that integument may be ; it would, therefore, be very 

 interesting to prosecute observations on this point regarding Cithna. In his generic diagnosis of 

 Lacuna (Brit. Conch., vol. iii. p. 343) Dr Jeffreys does not mention the epidermis. 



In the umbilical groove there is a minute difference between Lacuna and Cithna. In the former 

 a careful examination shows that the furrow (and the axial perforation where it exists), run strictly 

 within the labial edge ; in Cithna, on the other hand, the groove is external to the lip, and is a true 

 part of the umbilicus. This feature is very distinct in Lacuna divaricata (Fabr.), while in Lacuna pal- 

 lidula (Da Costa), it is more doubtfully recognisable. I am not aware what relation this feature of 

 the inner lip and umbilicus has to the economy of the animal's life, but it would seem to be one of 

 some importance, possibly in connection with the eggs, and in that case it is deserving of generic 

 recognition. 



The observation of the cfliated tentacles we owe to Dr Jeffreys. Forbes and Hanley (Brit. Moll., 

 vol. iii. p. 66) quote Dr Johnston as saying that Lacuna vincta (Mont.) ( = divaricata, Fabr.), has its 

 " tentacula setaceous," but it would appear that he only meant by setaceous shaped like a bristle, not 

 ciliated, as one might have supposed. It is unfortunate that in the Brit. Conch., vol. iii. pi. viii. 

 fig. 2, the artist has represented Lacuna with ciliated tentacles. 



Species. 

 1. Cithna tenella, Jeffr. 2. Cithna margaritifera, Wats. 



1. Cithna tenella (Jeffreys). 



Lacuna tenelta, Gwyn Jeffreys, Brit. Conch., voL v. p. 204, pL ci. fig. 7. 



Hela tenella, Gwyn Jeffreys, Mediter. Moll., Ann. and Mag. Nat. Hist., 1870, ser. 4, vol. vi. p. 12. 



,, „ Monterosato, Enumerazione, p. 23. 



„ „ Seguenza, Form. Terz. Calabria, pp. 268, 356. 

 Cithna tenella, Jeffreys, MolL "Lightning " and "Porcupine," Proc. Zool. Soc. Lond., 1883, p. 110, sp. 1. 



