FOSSIL SQUALIDiE OF THE UNITED STATES. 141 



of Professor Owen, of London, who has given to the world a splendid result of 

 laborious and scientific study in his Odontography.* 



The family of Squalidcs, which it is proposed here to illustrate, belongs to the first 

 order of Agassiz, denominated Placoids (from ?t?^a|, a broad plate). The skin is 

 irregularly covered with enamelled plates, sometimes large, but often in the form of 

 small points, forming shagreen in sharks, and tubercles in rays. Of these families 

 no remains are found in a fossil state other than teeth and vertebrae ; thouf>"h an 

 exception should be noticed in the discovery of the mouth of an Hyhodus, lately 

 reported by Sir Philip Egerton, from the secondary of the Isle of Wight, in which 

 the cartilaginous alee were traceable, and a part of the anterior cranial cavity.f 



The SqiialidcE constitute a large portion of the fossil remains of Fishes, and are 

 confined to the secondary and tertiary formations. An interesting observation of 

 Agassiz's is here worthy of notice. 



" De la comparaison des especes fossiles avec les especes vivantes, il resulte un fait 

 bien curieux, conforme a ce que I'etude du developement genetique du regne animal 

 nous apprend de tous les groupes bien etudies, c'est que les types generiques qui 

 prevalent dans la creation actuelle, ou n'ont pas de representans parmi les fossiles, ou 

 bien sont limites aux terrains tertiares et cretaces; tandis que les genres qui paraissent 

 isoles dans notre epoque, comme les genres Mustelus et Cestracion, sont represente 

 par de nombreux geiires analogues dans toute la serie des terrains secondaires.''} 



Notwithstanding the differences we observe in the many forms of teeth of sharks, 

 they all possess one essential character of structure, namely, a base or osseous root, of 

 variable form, fixed in the integument, and a crown or exposed portion projecting 

 into the mouth, covered with a greater or less thickness of enamel, assuming many 

 modifications by which the genera are characterized. These teeth only adhere to 

 the integuments and the covering of the jaw, and possess great mobility. They are 

 usually in rows, of which the anterior having been used fall out and are replaced by 

 others ; and new teeth are constantly forming within to succeed the outer as they are 

 lost. The base of these teeth is large and wide, rounded and hollowed or grooved, 

 but never conical nor terminated in acute points ; the root is osseous, more or less 

 compact or spongy, without any inner cavity. The crown is variable in form and 

 size in different genera, and even in different parts of the same jaw. In some which 

 are subulate and more or less triangular and compressed, those in the anterior portion 

 of the jaw are straighter and sharper than those in the posterior parts, which are 

 oblique and obtuse. There are marked differences sometimes in the teeth of the 



* Odontography : or a Treatise on the Comparative Anatomy of the Teeth ; their physiological relations, mode of 

 developement, and microscopic structure; illustrated by upwards of one hundred and fifty plates. By Richard Owen 

 F. R. S., &c. London. 1845. 



t Quarterly Journal of the Geological Society, London, vol. i, p. 198. | Poissons Fossiles, vol. iii. p. 75. 



