REPORT OX THE PTEROPODA. 
15 
the restricted sense of Heterofusus), principally on the ground of the transverse strise 
(at right angles to the axis), which are found in Lirnacina helicina. But this character 
cannot be regarded as of the value of a generic distinction. If we turn for instance to 
a group but slightly removed from the Limacinidae, the species of Clio of the subgenus 
Creseis, we see that Clio chierchiae, Boas, also possesses these transverse strise which are 
wholly absent in the other three species of the same subgenus. Yet one would not on 
that account dream of establishing a generic distinction on that simple fact, and a 
fortiori one cannot separate Lirnacina (s. str.) from “ Spirialis.” 
As to the genus Peraclis, Forbes, it is so distinct that it must be retained, although 
d’Orbigny has referred its typical species to Heliconoides, Souleyet and A. Costa to 
Spirialis, and Gray, Jeffreys, and Boas to Lirnacina. 
Peraclis differs indeed from the genus Lirnacina (as this has been defined above) in 
having a shell which is not umbilicate, has a few whorls ascending very rapidly, a larger 
aperture, a columella prolonged into a rostrum twisted into a spiral, and, further, in 
possessing a subcircular operculum, with a multispiral, left-handed coil. To this 
operculum neither d’Orbigny, Souleyet, nor Boas have attached the degree of import- 
ance demanded by its peculiar structure. But even if we do not take account of these 
differences, the structure of certain portions of the animal of Peraclis separates it 
markedly from all other Limacinidae, as we shall afterward see, and necessitates the 
formation of a distinct group, opposed to all the rest of the family. 
From the foregoing it results that there are among the living Limacinidae only two 
different genera, Lirnacina and Peraclis, which may be readily distinguished by turning 
to the synoptic table of genera (p. 8). 
Lirnacina / Cuvier. 
1817. Lirnacina, Cuvier, Le Regne animal, t. ii. p. 380. 
1823. Heterofusus, Fleming, On a reversed species of Eusus, Mem. Wern. Soc., p. 498. 
1824. Spiratella, de Blainville, Mollusques, Diet. d. Sci. Nat., t. xxxii. p. 284, iv. p. 
1836. Heliconoides, d’Orbigny (pars), Voyage dans l’Amerique meridionale, t. v. p. 174. 
1840. Spirialis, Eydoux et Souleyet (pars), Description sommaire de plusieurs Pteropodes 
nouveaux ou imparfaitement connus, Rev. Zool., t. iii. p. 235. 
1842. Helicophora, Gray, Synopsis of the contents of the British Museum, p. 59. 
1844. Scxa, Philippi, Fauna Molluscorum utriusque Siciliie, p. 164. 
1861. Protomedea, G. O. Costa {pars), Microdoride Mediterranea, p. 73. 
1869. Embolus, Jeflreys, British Conchology, vol. v. p. 114. 
Shell umbilicate, with turns gradually increasing ; with a fairly large aperture ; and 
with a columella not prolonged into a rostrum ; surface smooth or striated. The height 
of the spire, the form of the surface and that of the aperture, and the size of the 
1 Diminutive of Limax. 
