ARGTOPE. 15 



arched or reniform sinus ; the cardinal process is large, and the margin of the valve is 

 minutely granulated. 



Obs. This is the only British species of cretaceous Thecidea with which we are 

 acquainted, and is known to us since 1847, at which period we had obtained specimens 

 from Mr. Purdue, who found them while washing some chalk from Northfleet. It 

 has likewise been discovered by Mr. Wetherell, and lately described and figured by 

 Mr. Morris. 1 This species bears much external resemblance to Th. Mediterranea, and 

 more especially to Th. triangularis, figured by me from the inferior Oolite and Lias in 

 Part III, but quite distinct from both by the internal arrangements of the Apophysary 

 system, which partakes most of the simplicity seen in Th. Mppocrepis, but the dissepiment 

 is not so wide in that species. It may likewise bear some resemblance to Th. Broderipii, 

 of Michclotti; 2 but the figures of that shell are not complete enough to enable one to 

 compare the internal disposition of the Apophysary system. 



At the time Mr. Morris described this species, it was only known from the Upper Chalk 

 of Gravesend or Northfleet, Kent, where it abounds ; attached principally to Echino- 

 dermata, Inoceramus, and other shells ; the smaller valve being almost always wanting, 

 but the larger one, from being attached by the greater portion of its surface, is found 

 adhering to the object on which it lived ; at times it is assembled in great numbers, nor is 

 it rare to find Ananchytes with from twenty to thirty specimens fixed to them. During my 

 examination of the rich and interesting cretaceous collections assembled with great care by 

 Messrs. Cunnington and Faulkner, of Devizes, I at once recognised the species under 

 consideration obtained by these gentlemen in the Chalk of Pewsey (Wilts.), where it is most 

 beautifully preserved, with both valves united in the free state, owing no doubt to their 

 having been only slightly attached during life to perishable bodies, such as sponges, &c.; it 

 is also clearly seen, that they were not always fixed by the whole or greatest part of their 

 larger valve, as in the Gravesend specimens, but only partially so, as is illustrated by our 

 figures 20, 21, 22, 23, 24, 25, 26. I may also remark, that, as in Crania, in some cases 

 the ornaments or markings on the object of attachment are reproduced on the valves of the 

 Thecidea. It is also singular, that we should find in England no other species, so many 

 being met with in the foreign cretaceous beds. 



Plate I, fig. 15. Natural size. From the Upper Chalk of Gravesend. 

 fig. 10. Interior of unattached valve magnified. 



1 Mr. Morris's figures of this species are not very fortunate. From the artist not understanding the 

 character of the genus, he overlooked some of the most important points of structure ; thus, in the figure of 

 the larger or attached valve, the area and deltideum are not sufficiently characterised ; the teeth are wanting, 

 and the three small lamellar processes, as well as the granulation, incorrectly illustrated. In the figure of 

 the interior of the smaller valve, there exists in the specimens a small hridge-shaped process near the hinge, 

 marked b in my fig. 16, but omitted in those given by Mr. Morris. We have endeavoured, in our 

 numerous illustrations of this species, to make up for these deficiencies. 



2 Michelotti, ' Desc. des Foss. des Ter. Miocene d'ltalie,' Sept. 1847, pi. i, fig. 26. 



3 



