REPORT ON THE PTEROPODA. 
75 
was the first 1 to note the affinities of “ Cleodora curvatci,” Souleyet, to this group, but 
he did not detect what these four forms really represented, and regarded “Pleuropus” 
as a group within the genus Clio. 
The adults of most of the species of Pleuropus are known. As to the others, it is 
possible to predict, from some of their features, what forms they will probably turn out 
to be when arrived at sexual maturity. The designation Pleuropus is therefore to be 
abandoned. 
III. Diacria is a characteristic conckological genus. Gray erected it for the reception 
of Cavolinia trispinosa and two young stages of typical Cavolinia forms (group B of 
Boas), viz., Hyalsea depressa, d’Orbigny, and Hyalsea laevigata, d’Orbigny, which it would 
have been more natural to place beside Pleuropus. He leaves in the genus Cavolinia, 
Cavolinia quadridentata, though it is in all respects the neighbour of Cavolinia 
trispinosa. And, further, he places the same species ( Cavolinia orbignyi, Rang, fossil) 
both in the genus Diacria and in the genus Cavolinia . 2 
On the other hand, the brothers Adams , 3 and others after them, take this title 
Diacria as synonymous with Pleuropus, 1 and therefore add to Cavolinia trispinosa and 
to the two forms Hyalsea depressa and Hyalsea laevigata all the other young forms 
regarded as independent species. At the same time they agree with Gray in leaving 
Cavolinia quadridentata, separated from Cavolinia trispinosa, beside the typical 
Cavolinia forms . 5 
Now, it is certain that if Cavolinia trispinosa is to be separated from the other 
species of Cavolinia, Cavolinia quadridentata must go with it. The two species are in 
their structure most closely allied, and form a well-defined subgroup contrasting with the 
six other species. 
And if, in their embryonic shell, in the form of their fins, and in the posterior 
portion of the foot, they present resemblances to Clio [Cleodora), they at the same time 
exhibit the characteristic features of Cavolinia in a way that makes separation impos- 
sible. They are certainly the most archaic living forms of the genus, but not sufficiently 
distinct to warrant a separate genus. One may, however, follow Boas in establishing a 
subsection ( Hyalsea , A), within the genus Cavolinia. 
IV. Orbignyia, which was only regarded as a subgenus by A. Adams, is based on 
Cavolinia injlexa, which is usually considered as allied to Clio [Cleodora). There is, 
1 Catalogue of the Mollusca in the Collection of the British Museum, pt. ii., Pteropoda, p. 14. 
' l This Catalogue is in other respects full of inaccuracies and carelessness. It would he desirable to re-edit it, 
especially since the collection of Pteropods in the British Museum is many times richer to-day than it was in 
1850. 
3 The Genera of Recent Mollusca, vol. ii. p. 611. 
4 Similarly Pfeffer, Uebersicht der auf S.M. Schiff Gazelle und von Dr. Jagor gesammelten Pteropoden, 
Monatsber. d. Jc. preuss. Alcad. d. Wiss. Berlin, 1879, p. 236. 
5 Pfeffer (Die Pteropoden des Hamburger Museums, Abliandl. d. Naturiv. Ver. Hamburg, t. vii.) places Cavolinia 
trispinosa in the subfamily Cleodorinee, and Cavolinia quadridentata in the subfamily Hyaleinae. 
