£>R RAMSAY H. TRAQTJAIR ON THELODUS PAGET. 597 



from the Silurian of England and of Russia, # and, on more perfect examples being found, 

 it turned out that their contour of body was identical with that of the Cephalopterus or 

 Turinia. This led to a critical re-examination of the Forfarshire specimen, which, first 

 convincing me of its close alliance with Thelodus, left me finally no choice but to merge 

 it in the same genus. 



Thelodus parvidens was the name given by Agassiz in 1839 to certain minute 

 bodies, which are abundant in the well-known Ludlow " Bone Bed," on the supposition 

 that they were fish-teeth resembling those of Lepidotus. These bodies, " small as grains 

 of fine sand," are of a somewhat stud-like form, that is to say, constricted below the top, 

 which is smooth and flat, or slightly convex, while in the centre of the under surface of 

 the base is seen the opening of an internal pulp-cavity.t 



That these bodies were not, however, teeth, but scales, or "the earthy grains or 

 shagreen of the skin of large cartilaginous fishes," was maintained by M'Coy, not only 

 because of the abundance with which they sometimes occur over patches of rock, but 

 also on account of their microscopic structure. He suggested that they were in all 

 probability the shagreen granules of the same fish as bore the spines known as 

 Onchus tenuistriatus, Ag., fragments of which are common in the same " Bone Bed."| 



Murchlson, in his Siluria (1854, p. 238), adopted M'Coy's views as to the correla- 

 tion of Onchus and Thelodus in the following words : — 



" Onchus tenuistriatus and Murchisoni are the bony fish defences such as were 

 possessed by many placoid fishes of the old rocks. ... The small cushion-like bodies, 

 called formerty Thelodus parvidens by Agassiz, and which occur in myriads in the 

 stratum, often forming large portions of its thin layers, are certainly the granules of the 

 skin, or shagreen of one or other of these two common species." 



In his well-known work on the Silurian Fishes of Russia,^ Pander instituted the 

 family Ccelolepidse for various minute scales, or shagreen-bodies, from the limestones of 

 Oesel, and of which he made three genera, — viz., Ccelolepis, Pachylepis, and Nostolepis, 

 which were to be distinguished by the relative size, or, in the last case, by the absence, 

 of the opening on the under surface of the base of the scale. Recognising the close 

 resemblance of his Pachylepis to Thelodus of Agassiz, Pander stated his readiness to 

 withdraw the former name, if the identity of the two should afterwards come to be 

 fully proved ; though in that case he proposed to change Thelodus into Thelolepis. I 

 do not find any indication of Pander having believed in the correlation of the Onchus 

 spines with the minute scales which he included under the term Ccelolepidse. 



However, Prof, von Zittel, writing in 1887, placed Thelodus in the sub-order 



* Detached scales have also occurred in the Upper Devonian of Russia, and have been described and figured by 

 Rohon as Thslolepis ( = Thelodus) Tulensis. 



+ In Murchison's Silurian System, vol. ii., 1839, p. 606, pi. iv., figs. 34-36. 



X Qu. Journ. Geol. Soc, vol. ix. (1853), p. 14 ; British Palaeozoic Fossils, p. 576. 



j G. H. Pander, Monographie der Fossilen Fische des Silurischen Systems do- Rttssisch-baltischen Gouvernenurtts, St 

 Petersburg, 1856. 



