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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Jan. 23, 



from the list of Dibranchiate Cephalopods, and consequently that the 

 pakeontological history of this group cannot, at present, be traced 

 back further than the beginning of the Mesozoic epoch. 



The distinction of species among the Pteraspides is a difficult 

 matter ; and, pending investigations which I have been for a long 

 time making on this subject, I leave open the question whether 

 Prof. Roemer's specimens are or are not types of a new species, 

 which, in the latter case, must be termed Pteraspis Dunensis. 



In conclusion, I may remark that, as I have already pointed out 

 elsewhere (British Association Eeports, 1858), the test of Pteraspis, 

 as commonly met with, consists of only a part of the cephalic shield 

 of that singular fish, the whole shield being not a little similar to 

 that of Cephalaspis. In Pteraspis rostratus, for example, the entire 

 shield has the form indicated by the subjoined outlines, of which A 

 represents a dorsal, and B a lateral view. It consists of a cephalic 

 rostrum (a), more or less elongated and pointed according to the 

 species, passing posteriorly into the broad shield (b), which (as 

 the dotted lines indicate) is commonly found broken off and alone. 

 When perfect, this is produced laterally and posteriorly into two 

 comua (c), and in the middle line behind passes into a broad 

 prolongation (cZ), which gives rise interiorly to a long, curved, and 

 backwardly produced spine (<?). Upon each side of the test, where 

 the rostrum joins the rest of the shield, there is a round well- 

 defined aperture (/), which may be either the orbit or the nasal 

 aperture. 



It is not easy to find an exact parallel for such a cephalic covering 

 as this among existing fishes. Loricaria, Tetrodon naritus, Acipenser, 

 and Spatula ria seem to present the nearest analogies, — the two 

 former being much more remote than the two latter. In fact, if the 

 bony cephalic shield of the Aeipenseroid fishes were ossified in one 

 piece, it would very closely resemble that of both Cephalaspis and 

 Pteraspis, and would hardly (lifter more from either than the two 

 from one another. 



3. On tJie " Chalk-rock," the Topmost Bed of the Lower Chalk, in 

 Berkshire, Oxfordshire, Buckinghamshire, &fc. By "William 

 Whitaker, B.A., F.G.S., of the Geological Survey of Great 

 Britain. 



The bed to which I have elsewhere* given the name of "Chalk- 

 rock/' I believe to form the division between the Upper and Lower 

 Chalk, and to be the topmost bed of the latter. I have described it 

 as "hard blocky chalk, jointed perpendicularly to the plane of bedding, 

 with lines of irregularly shaped, hard, calcareo-phosphatic nodules, 

 which are green outside, but cream-coloured within." It breaks 

 with an even fracture, rings when struck with the hammer, and is 

 of a pale cream-colour (the nodules being darker than the rest). 



* Catalogue of Rock-Specimens in the Museum of Practical Geologv, 2nd 

 edition, p. 296. 



