Palaeontology. 71 



this fin distinctly exhibits on its posterior margin the remains of cartila- 

 ginous rays that bifurcate as they approach the edge of the fin, shewing 

 in this respect a new approximation to the fin of a fish, and more fully 

 justifying the propriety of the name Ichthyosaurus. Traces are also pre- 

 served of scutiform compartments on the integument of the fin. It is 

 singular that this structure should never have been observed in any of the 

 numerous specimens from Dorset and Somerset that have come under our 

 notice ; whilst at Barrow-on-Soar, from whence the paddle in question 

 was derived, even the fibres of the skin and folds of the epidermis are 

 sometimes accurately retained.* 



Mr Owen's first part of his report on fossil Saurians, read at the British 

 Association at Birmingham in August last, forms the commencement of a 

 most important addition to the history of extinct reptiles. His recent 

 investigations in odontography have also supplied to the geologist a new 

 and most efficient instrument of investigation, enabling him to distinguish 

 genera of extinct animals by the microscopic structure of their teeth ; and 

 as, of all fossil remains, the teeth are the parts most perfectly preserved, 

 and in the case of cartilaginous fishes the teeth and spines are generally 

 the only parts that hare escaped decomposition, this method assumes an 

 especial importance in fossil ichthj-ology, as affording exact character- 

 istics of animals long swept from the surface of the earth, and whose very 

 bones have been obliterated from among the fossil witnesses of the early 

 conditions of life upon our planet. By this microscopic test applied to 

 the family of sharks, Mr Owen has confirmed the views of Agassiz, re- 

 specting the affinities between the living cestracion and the extinct ge- 

 nera Acrodus, Ptychodus, Psammodus, Hybodus, Cochliodus ; in the 

 case of animals also of the higher orders, he has settled the much-dis- 

 puted places of several extinct gigantic mammalia by the same unerring 

 test. Thus he has shewn the supposed reptile Basilosaurus to be a ceta- 

 ceous mammifer, allied to the dugong ; the Megatherium to be, as Cuvier 

 had considered it, more nearly allied to the sloth than to the armadillo ; 

 and the Saurocephalus to be, as Agassiz had supposed it, an osseous fish. 

 Dr Malcohrison, in a memoir on the old red sandstone of the north of 

 Scotland, has done important service in shewing that the rocks composing 

 that group are divided into three formations, the two lower of which are 

 clearly distinguished from each other by their fossil fishes. The corn- 

 stone or central formation is charged with numerous remains of iehthy- 

 olites, including Holoptychus nobilissimua, a new species of Ccphalaspis, 

 and other forms not yet described. The lower division, consisting in this 

 r region of conglomerates, shales, and sandstone, is characterized by the 

 genera Dipterus, Diplopterue, Cheiracanthus, &c. of Agassiz, as well as by 

 the occurrence of a singular iehthyolite, which seems to offer close ana- 

 logies to certain forms of Crustacea. By help of these iehthyolites, the 



See Buckland'a Bridirewator Treatise, PI. 10. 



