internal Structure of the Magnesian Limestone. 117 



me to complete the series, and to procure figures, more or less perfect, of 

 perhaps all the species which have yet been discovered. 



It was evident from the first, that many of these fossils, notwithstanding- 

 their mutilated condition, very nearly resembled the celebrated fish of the 

 copper-slate of Germany. By far the greatest number came unequivocally 

 under the genus Palaeothrissum, being of the order Malacopterygii abdomi- 

 nales, having a forked tail, the rays proceeding exclusively from the lower 

 border, the upper lobe being longer than the lower, and covered with scales, 

 and having one dorsal fin between the anal and the ventral. It mi^-ht be 

 added, that all the species possessing the above characters appear to have had 

 strong hard scales, which were partly tessellated and partly imbricated, and 

 arranged in oblique rows ; in which respect they make some approach to 

 Esox osseus. 



As there were, however, considerable difficulties in making out the specific 

 characters of these fossils, some of the earliest specimens were submitted to 

 the examination of M. de Blainville ; and that celebrated naturalist, as was 

 before stated, identified two of them with the species Palaothrissum magnum 

 and Palceothrissimi macroccphalum, which he had himself previously esta- 

 blished from the ichthyolites of Mansfeld*. 



The following plates of fossil fish found in the marl-slate of Midderidge and 

 East Thickley, are as nearly as possible of the natural size, and will it is hoped 

 convey as correct a notion of the several species as their state of preservation 

 allows. 



1. PaloEothrissum magnum \. — This specimen shows the position of the fins ; also the arrange- 

 ment and form of the scales, some of which, on the fore part of the body, exhibit a kind of im- 

 perfect articulation, as is represented on a magnified scale under the Plate. The character of 

 the tail is well seen ; but it is mutilated. When perfect, it is nearly of the same shape as in the 

 next species. 



2. Palwothrissum macrocephalum. — This (like the former species) is very abundant. It has 

 many characters in common with the P. magnum ; differing, however, greatly from that species 

 in its size, and in the form of its head. The individual here figured is a large specimen +. 



3. PalcEothrissum elegans. — This is a much more rare species than the preceding. The head 

 is smaller; and the upper and lower portions of the forked tail are much more nearly equal §. 



4. Another species ||, in the form of the tail, the position of the fins, and the arrangement of the 



* The genus Palffiothrissum was formed by M. dc Blainville out of several species of fiih 

 which he had found exclusively in the coppcr-slatc. '^11 a pour curactere csscntiel: dU/rc 

 abdominal., malacopterygicn^ de n^avoir qi/Ucne scale nagcoire supericure siluce avant 

 I'unale, entre les pelviennes et elle., et surtout d'avoir la (pieue O/furquic, et k lube supcricur 

 ordinuirement beaucoup plus long (pie I'iiiJ'crieur^ ct convert d'ccaillcs dans tontc sa moitiv 

 superieure.'" See the article on Ichthijolitcs, Nouveuu Dicteonnaire d'JJis/uire Na/urellc, 

 vol. xxviii. 



t Plate VIII. fig. 1. t Plate IX. fig. 2. § Plate IX. fig. 1. || Plate VIII. fig. 2. 



