484 New Genus of Land Shells. 



placed at my disposal by Dr. Shearman, has contributed to 

 resolve that question.* This foetus {fig. 59.) was farther ad- 

 vanced than the one which I previously described. The objects 

 on the hinder extremities were distinctly formed. The umbilical 

 chord extended nearly three lines from the abdominal surface 

 of the foetus; the amnios was reflected from this point, to form 

 the usual immediately investing tunic of the foetus ; and, be- 

 yond the point of reflection, the chord divided into a very 

 large superior vascular sac, organised by the omphalo-me- 

 senteric vessels, corresponding in all respects with the vitelline 

 sac described and figured in my first paper ; but below the 

 neck of this sac there extended a second pyriform sac, about 

 one sixth the size of the vitelline sac, having numerous rami- 

 fications of the umbilical vessels, and constituting a true al- 

 lantois. This sac was suspended freely from the end of the 

 umbilical chord : it had no connexion, at any part of its cir- 

 cumference, with the chorion, and was equally free from attach- 

 ment to the parietes of the uterus in which the foetus was 

 developed. 



Art. VIII. On a netv Genus of Land Shells. By John Edward 

 Gray, Esq., F.R.S., &c, 



Zoologists have divided the land shells into several genera ; but the 

 late Baron Ferussac united most of them into a single genus, as he wished 

 to establish as a rule, that all the genera of Molluscans should be alone 

 characterised by some peculiarity in the animal. 



The increased knowledge of the animal has shown that some of the 

 species which he referred to the genus Helix have very different animals 

 from the typical kinds ; and it is probable that eventually several of the 

 genera established before his time (which he attempted to set aside) 

 will be found to be true genera, according to his own theory. 



The knowledge of the animal, and the history of several species which 

 were unknown at the time he wrote his system, has shown that several of 

 the characters which he considered as of generic importance are common 

 to other species belonging to quite different groups. Thus, we now 

 know that some Helices (Caricolla inversicolor, Balea Chemnitzii, and 

 some others) are viviparous, as well as the Partulas; that the degree of 

 developement in the lower pair of ventricles is variable in the different 

 species of Pupae and Vertigo; and that to separate the latter genus from 

 the former, on account of the partial obliteration of these organs, has the 

 effect of dividing very nearly allied species. 



I am inclined to think that these, and numerous similar facts, which 

 must be well known to every practical conchologist, show us that we are 

 warranted in establishing genera from any peculiarity in the structure or 



* The preparation from which Mr. Owen's drawing for our cut was 

 taken, was exhibited by him at the last Scientific Meeting of the Zoologi- 

 cal Society, August 22d, when the facts contained in the present paper 

 were orally communicated. — Ed. 



