14 
THE VOYAGE OF H.M.S. CHALLENGES. 
1. Heterofusus, Fleming (1823), and Scaea, Philippi (1844) ; the former based on 
Heterofusus retroversus, Fleming, the latter on Scsea stenogyrci, Philippi. But as these 
two species are identical, the two generic titles are absolutely synonymous, and the more 
recent ought to disappear. 
2. Protomedea, 0. G. Costa (1861), and Embolus, Jeffreys (1859) ; the former based 
on Protomedea elata, 0. G. Costa, and the latter on Atlanta injlata, d’Orbigny. But as 
the two species are synonymous, the two generic titles are equally so ; and since the 
name Protomedea was applied in 1834, by de Blainville, to a Coelenterate, it ought to 
disappear. 
3. Peracle, Forbes (1844), Campylonaus, Gray (1847), and Euromus, A. and H. 
Adams (1858) : the two last based on Atlanta reticulata, d’Orbigny ( = Spirialis clathrata, 
Eydoux and Souleyet). and the first on Peracle physoides, Forbes. But as these two 
specific types are now recognised to be identical, the three generic titles are obviously so 
too, and the two more recent ought to be disused. 
Having reached this stage of our critical review, we see that the maximum number 
of generic titles which can be adopted for the Limacinidse does not exceed those four — 
Limacina, Heterofusus, Embolus, and Peracle — since we may abstract Heliconoides 
( = Spirialis — Helicophor a), this genus being succeeded by the three generic titles 
referring to the three forms of shell which it includes. 
But are Heterofusus and Embolus really distinct, with this simple difference, that in 
the second the spire is depressed and the lip rostrate ? This can hardly be, for in 
almost all the genera of Gastropods there are species with short and others with elongated 
spirals, and the same is true of the rostrate lip. Thus in a group adjacent to the 
Limacinidse, the important genus Clio ( = Cleodora) exhibits nearly related species, some 
with a rostrum on the dorsal surface and others without. Nevertheless these forms are 
much too closely allied to be generically separated, and ought not the same to apply to 
Heterofusus and Embolus ? Both exhibit in fact an umbilicate shell, with whorls 
increasing somewhat gently, and a semilunar operculum, with a right-handed spiral of 
few turns ; nor do the animals exhibit any difference in their structure. 
But besides having these characters in common, they share them with Limacina, 
from which they do not differ in any character sufficient to establish a generic distinction, 
although, as I have already noted, the reverse has been maintained by Gray, Gould, Boas, 
and to a certain extent by Jeffreys. It must be remarked on the other hand that 
Souleyet, who created the genus Spirialis (including Heterofusus and Embolus), 
recognised that it ought to be united with Limacina if there were an operculum in the 
latter . 1 But it is now sufficiently demonstrated that in Limacina an operculum does 
exist. 
It is true that Sars maintains the generic distinction of Limacina and Spirialis (in 
1 Voyage de la Bonite, Zoologie, t. ii. p. 211. 
