342 Dr. Traquair — On Psephodus magnus, from E. Kilbride. 



Among these considerable variety of form may be observed. One 

 at least (/) is referable to L. didymus, Ag. sp., showing a broad 

 prominent central elevation or boss, which " posteriorly " is distinctly 

 divided into two smaller ones, below which and close to the margin 

 of the crown is a small angular prominence. Though the division 

 of the central elevation of the crown is feebly indicated in at least 

 one of the others, in the rest it disappears, and the teeth assume the 

 characters of L. loemssimus, Ag. sp. (g) ; finally, all definition of the 

 boss ceases, the crown ultimately becoming nearly flat. In some of 

 these "Icevissimus " forms the boss is markedly unsymmetrical, being 

 much more sharply defined on one side than on the other. 



Concerning the rest of the specimen not much is to be said. 

 Immediately behind the large tooth (a) there is a flat piece of 

 osseous matter ^^ inch in length by -f in breadth, concerning which 

 I am unable to offer any opinion. The remains of the vertebral 

 column, of which a length of three inches is preserved, are limited 

 to a set of elongated closely-placed spinous processes, which are, 

 however, very obscurely seen ; there is no trace of vertebral bodies, 

 so that, as might be expected in a Palaeozoic Selachian, the notochord 

 must have been persistent. One or two minute, sharply-ridged, 

 shagreen bodies may also be observed. 



Observations. — Although the above-described specimen is one of 

 great importance for Carboniferous Selachology, our first reflection 

 must be that we are far from having the entire dentition of the 

 fish before us, and that a considerable portion of the specimen must 

 have been lost. It is very improbable that the species possessed 

 less than four of the large tooth plates — one on each side above 

 and below — but were only one pair present the two would be 

 symmetrical with each other. Here we have two of these plates 

 present, belonging respectively to the forms supposed by M'Coy and 

 Davis to be the first and second tooth of a cochliodont dentition, but 

 the mode in which they occur lends no support whatever to that 

 theory. Much more probable does it seem to me that the view of 

 Messrs. St. John and Worthen is correct, namely, that these two 

 forms of teeth belong to different jaws, upper and lower. Which 

 belongs to which we have, however, no really accurate means as yet 

 of determining. Messrs. St. John and Worthen refer the broader 

 teeth (b of the present specimen) to the lower jaw on account of 

 " the somewhat stronger resemblance " which they have in form 

 " with the contour of the mandibular teeth of typical cochliodonts." 



But whether the form («) or the form (6) be upper or lower, I 

 must hold that these larger tooth plates in Fsephodus present an 

 unmistakeable analogy in form with those of CocMiodus. And if this 

 be the case, the long side of each along which the progressive 

 increase of the tooth took place as indicated by the lines of growth, 

 must be internal in position, and not external as it is placed by Mr. 

 Davis in his figures. This position also brings the oblique directions 

 of the undoubted anterior and posterior margins into harmony with 

 those of the anterior and posterior margins of the great posterior 

 tooth of CocMiodus, and of the lines separating the rows of teeth 



