17 



is stronger and not directed outwards. Nor is "-Spirialis diversa" Monterosato ') identical with 

 one of tlie above-named forms. 



The smoothness of the shell shows that the reticulation does not occur always among 

 the species of this genus. 



Anatomical Notes. 



The organisation of the genus Peraclis, being known by a single representative (P. 

 reticulata) until now, has only been studied by Pelseneer -). The differences from Limacina 

 are very striking, and these differences connect Peraclis with the Cymbuliidae. This is a fact of 

 great importance and of the utmost value, as regards the phylogenetic affinities among the 

 genera of the Thecosomata ^). 



I shall restrict myself to my own researches. 



The head is situated on a short proboscis fPl. I, fig. ii), rising from the dorsal margin 

 of the fins; the lips unite ventrally, as well as dorsally, and have thickened walls; tentacles 

 very small, symmetrical, situated on the same transverse line, and without sheath. 



Foot: The posterior lobe of the foot is short, with broad base, and scarcely notched 

 in the middle of its free margin. The fins are broad, without tentacle-like lobe; at their distal 

 extremity they exhibit a shallow sinuosity; the ventral margin of the fins is situated more 

 distally than the dorsal one, the inclination of the swimming organs with regard to the body 

 therefore differing about 90° from that of Limacina. 



Mantle: The pallial gland exhibits transverse bands, and was found to be symmetrical*). 

 In the shield, seen from within, I noticed an elongated organ, which is perhaps the heart. On 

 the right side of the mantle rises a long "balancer", whose insertion is somewhat pyriform. 



(The description of the mantle refers only to Peraclis rissoides; other forms I have 

 not examined in this respect). 



Digestive Tract: There are two salivary glands, visible by a whitish colour on the 

 brown oesophagus. At the entrance of the digestive tract I noticed two lateral jaws, especially 

 developed towards the ventral side. Each jaw consists of four transverse series of horny teeth ; 

 the teeth towards the median and ventral side carry numerous hooks, more obtuse and less 

 in number {^Peraclis reticulata var. minor, PI. I, fig. 12). The difference between the ventral 

 teeth and the lateral ones is most distinct on the first transverse row and becomes gradually 

 less distinct on the following series. Every row is inserted on a strong transverse muscular 

 band. To my opinion, the jaws of Peraclis differ from those of Limacina in being stronger ^). 



As to the radula, I have not noticed anything remarkable; it is quite the same as in Limacina. 



The oesophagus is extremely short. Further information may be found in Pelseneer's 

 work. The extreme smallness of the animals did not permit me to study their organisation at 



1) Nuova revista, etc., jMli .^c. sc. lett. ed arti, vol. V, p. 50. A good description with figures is given by Locard Exp. 

 sc. Travailleur et Talisman, Moll., vol. II, p. 29, pi. I, figs. 4 — 6. 



2) Challenger Rep. LXV'I, p. 11 — 12. 



3) Ibid., p. 28—37. 



4) P^LSENliER (op. cit.) remarks that the pallial gland is asymmetrical in Peraclis reticulata.! as in Limacina. 



5) Sars, Mollusca regionis arcticae Norvegiae, PI. XVI, fig. i^jb. 



SIBOGA-EXPEDITIE I.II. , 



