318 report — 1865. 



these, as in the foregoing species, the comparison of many individuals from 

 beds of the same general age occurring in the same district, illustrates the 

 wide range of variation which accompanies great fertility. Similarly, the 

 many specimens of the Mansfeldt species in the Royal College of Surgeons, 

 in the British Museum, in Sir P. Egerton's Cabinet, and in Edinburgh, 

 confirm the hesitation with which Geinitz enumerates them, and justify the 

 reduction of their specific differences within very narrow limits. The figures 

 of the same species given by him, by Agassiz, and by Xing, differ in general 

 proportions and the position of the fins, as markedly as do the Burdie House 

 forms, and in this respect fairly parallel the variations in the more limited 

 area of North Staffordshire. The comparison of individuals belonging to 

 prolific living species, as the Herring, Salmon, Trout, yields similar results. 

 The writer drew attention to the American specimens in the Geological 

 Society's collection. The species representing those of the English Carboni- 

 ferous series are associated with forms allied to those of Mansfeldt, whose 

 coexistence in Staffordshire has been asserted, but is not yet certain. 



Of the genus Rhizodopsis, established provisionally by Prof . Huxley /or the 

 reception of certain specimens whose cranial and dental characters approached 

 those of Rkizodus, Ow., while the scales, not yet found in unquestionable 

 relation to the crania on which the older genus was founded, belong to the 

 cycloidal section of the Glyptodipterini, several good examples occur. None 

 of the individuals reach the size of MTiizodus, with which they are coextensive 

 in distribution in the English and Scottish Coal-fields. The facial bones are 

 not determinable. The strong conical teeth with plicate bases are of two 

 sizes ; the larger, slightly incurved at the apex, want the lateral compression 

 which marks the laniaries of Bhizodus. Only two pairs of jugular jnates seem 

 to have existed. The pectoral fins are lobate ; the two dorsals, the ventral 

 and anal, opposite each other respectively, are placed in the posterior half 

 of the body. The vertebral rings are thin, shallow. The scales are cordate, 

 but vary in the proportions of their measurements at different parts of the 

 body. Their ornament consists of concentric and radiating ridges, the for- 

 mer entire on the sides, but interrupted at either extremity by the latter ; 

 the concentric ridges of the free area are robust. A subcentral boss on the 

 lower surface, nearer the anterior margin, corresponds to the point of radia- 

 tion on the upper aspect. The amount of overlap is uniform, extending to 

 one-third of the surface. These are the chief points in which this genus 

 differs from other Glyptodipterines. The character of the larger teeth justifies 

 its retention as distinct from Bhizodus. Reliable specific differences are 

 wanting ; the greater flexibility of the scales in specimens from shale than in 

 those from the ironstones, is due perhaps as much to the nature of the matrix 

 as to different stages of growth ; while greater and smaller size are not accom- 

 panied by other noticeable variations. 



Certain cycloidal scales were determined by Prof. Huxley in 1863 to belong 

 to a new genus which he named Cycloptychius. His description of the genus 

 was not then published, though read at the British Association. Several 

 excellent examples have since been found ; their structure is in abstract as 

 follows. 



Body : average length 5 inches, slender, tapering to a finely pointed caudal 

 extremity ; covered with thin scales, cycloid anteriorly, pointed posteriori}', 

 marked with well-defined concentric ridges. Head nearly one-fifth of length 

 of body, compressed ; muzzle rounded. Orbits far forward, bounded by large 

 supraorbitals, prefrontals, and inferiorly by a long suborbital bar. Praj- 

 maxilla large, its anterior margin prolonged laterally. Maxilla in one piece, 



