80 PERMIAN FOSSILS. 



the impressions just noticed ought to be immediately opposite to them. I can safely 

 assert, however, that this is not the case in Productus horridus, as I have now examined 

 a number of perfect casts of this species, and in all of them the latter are always 

 considerably behind the former. 



But perhaps no shell exhibits impressions of the labial processes so distinctly as 

 the remarkable valve figured by M. de Verneuil in the ' Geology of Russia' (vol. ii, 

 pi. xvi, fig. 9), in which the impressions, occupying its two lateral halves, are in the 

 form of two depressed cones, having their surface strongly marked with five or six 

 spiral grooves, gradually rising above each other from the base to the apex. 



A single other reference may be allowed before drawing the present subject 

 to a conclusion. The indisputable existence of horizontally-spirally-folded labial 

 appendages in Productus prevents my instituting a comparison between its reniform 

 impressions and the singularly complex furrows on the flat valve of Thecidaa 

 Mediterranea ; because, from the observations of M. Alcide d'Orbigny,Mt would appear 

 that this interesting Palliobranch does not possess any labial processes spirally folded ^% 

 in Productus. If M. d'Orbigny's view be correct,^ this shell possesses the homologues, 

 though certainly singularly modified, of the recurved labial appendages characteristic of 

 Ancylopods. 



Since the publication of my ' Remarks on certain Genera of the Class Pallio- 

 branchiata,' there have appeared in various journals some highly valuable communi- 

 cations,^ which have thrown so much important light on the classification of this 

 division of molluscous animals, that I have been enabled to make several corrections 

 and alterations in the following Synoptical Table, as compared with the one contained 

 in the above-named paper. 



As regards a Fundamental Classification of the group, I feel much pleasure in 

 adopting Mr. J. E. Gray's, as it is based on a modification of an important organic 

 structure, — a modification which is, at the same time, manifested by a corresponding 

 variation of the apophysary system. 



1 Vide Annales des Sciences Nat., Oct. 1847. 



2 I would suggest in all new examinations of Thecideea Mediterranea, that its furrows be compared with 

 the digitated crania-like impressions of Thecidcea digitata ; and that a comparison be made between the 

 organs occupying the furrows with the branchiferous vessels oi Lingula. 



3 It is necessary to mention that Mr. Morris's paper ' On the Subdivision of the Genus Terehratula' 

 (vide Quarterly Journal of the Geological Society, vol. ii, part ii, p. 382-9), in which he took nearly the 

 same view of the genus Hypothyris as the one published by myself, was communicated to the Geological 

 Society at the time my own paper in the 'Annals and Magazine of Natural History,' vol. xviii, 1846, 

 was going through the press. 



