REPORT ON THE PTEROPODA. 
33 
2. Lips with, lateral angles, and united ventrally. 
3. Tentacles symmetrical, of the same size and without sheath. 
4. Posterior or opercular lobe of the foot broad at the base, instead of being slightly 
constricted as in Limacina, and less developed in proportion to the fins, which are large, 
long, truncated at their distal extremity, and without the small tentacle-like lobe. 
5. Visceral ganglia forming three distinct masses, as in the Cymbuliidse. 
Observations. — I. Boas has made a mistake in figuring the operculum as twisted in a 
right-handed spiral. 1 The coil is left-handed, as d’Orbigny has represented it. 2 This 
arrangement is quite unique, for in all the operculate Mollusca the twisting of the 
operculum is in the opposite direction to that of the shell. Atlanta is the only right- 
handed Mollusc in which the operculum is coiled to the right. In all the left-handed 
operculate Molluscs the operculum is coiled to the right — Limacina , 3 Triforis , 4 Lseocochlis. 5 
Peraclis thus forms a remarkable exception. 
II. The initial portion of the spire does not project, so that the apex is always 
obtuse. 
III. Boas 6 notes in “ Limacina” reticulata ( = Peraclis) a small tentacle-like lobe on 
the fin as in Limacina helicina; this observation was made on an insufficiently preserved 
specimen, and has not been figured. I have examined not only the preserved specimen 
of the Challenger Expedition, but living specimens from the Mediterranean, and am able 
to state that the fin does not bear any lobe. Costa’s figures 7 are perfectly correct in this 
respect. Boas must have mistaken a fold of the fin margin for the lobe. 
D’Orbigny, the discoverer of the only species as yet known, considered it, as well as 
all the small forms of Limacina (the Spirialis of Souleyet), as Heteropods of the genus 
Atlanta. Forbes, who gave a second specific title to the form in question, and created 
for it the generic title Peracle, also regarded it as a Heteropod. Gray also regarded it 
as such, under the title Campylonaus. 8, 
Souleyet was the first to place this form, with a third specific title, among the 
Pteropods, but was unable to investigate the animal. Subsequently Costa figured the 
paired fins of the animal, to which he gave a fourth specific title, and made its position 
as a Pteropod indisputable. 
The structure of the genus has, however, remained quite unknown till now. I have 
been able to investigate it to some extent, and to show that it is of the highest interest 
1 Spolia atlantica, pi. iii. fig. 39. 
2 Voyage dans l’Amerique meridionale, t. v. pi. xii. fig. 39. 
3 Souleyet, Voyage de la Bonite, Zoologie, Mollusques, pi. xiii. ; Sars, Mollusca regionis arctic® Norvegise, 
pi. 29. 
4 Sars, Iiid., pi. xviii. fig. 31. 
5 Sars, Ibid., pi. xviii. fig. 29. 
6 Spolia atlantica, p. 50, note 2. 
7 Annuario del Museo Zoologico della R. TJniv. Napoli, t. iv. pi. iv. fig. 12. 
8 Proc. Zool. Soc. Lond., p. 149, 1847. 
(ZOOL. CHALL. EXP. — PART LXV. 1887.) 
Ttt 5 
