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 [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, 
pl. xvi, fig. 9), im 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 Thecidea 
Mediterranea ; because, from the observations of M. Alcide d’Orbigny,’ it would appear 
that this interesting Palliobranch does not possess any labial processes spirally folded as 
in Productus. Vf 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 Thecidea Mediterranea, that its furrows be compared with 
the digitated crania-like impressions of Thecidea digitata ; and that a comparison be made between the 
organs occupying the furrows with the branchiferous vessels of Lingula. 
3 It is necessary to mention that Mr. Morris’s paper ‘On the Subdivision of the Genus Terebratula’ 
(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. 
