Reviews — Prof. H. Crednev — on. the Stegocepliali. 83 



BrancMosauri up to tlie huge Sclerocephalus with almost inch-long 

 teeth. ^ Their variety in external appearance, as well as in the 

 extreme specialization of their skeleton, in which they diverge in a 

 more or less degree towards the Reptilia, is so extraordinary that 

 we have to look for their progenitors in still more remote periods. 

 On the other hand, the less varied teeth, studied in the present 

 Memoir on the dentition of Scleroceplialus lahi/rinthicus, Gein. sp., 

 are proved, by their histological details, to be ultimately the homo- 

 logues of the minute teeth of certain ichthyous scales, and at the 

 same time throw an unexpected light in an opposite direction, 

 inasmuch as we begin to have a clue as to the primitive form and the 

 evolution of the Vertebrate tooth in general. 



In the first chapter, the dentigerous bones of the buccal cavitj^ (viz. 

 the premaxilla, maxilla and mandible the anterior alas of the pterygoid 

 and the palatine), as well as almost all the bones of the skeleton, are 

 shown by their characteristic structure to be dermal bones, nearly 

 related to the Ganoid scales. The dermal bones of the cranium, 

 particularly, prove to be the exact homologues of these last, as they 

 are composed of the same three strata: (1) a thin basal stratum 

 of calcareous lamellse (BasalscJiicht von KaUdamellen), followed by 

 (2) a much thicker stratum of osseous tissue, which is distinguished 

 chiefly from that of the euchondral bones of the rest of the skeleton 

 (including the quadrate and ai'tioular of the mandibular articula- 

 tion) by a dense system of blood-vessels (Haversische Candle of 

 Klaatsch), extending in a horizontal direction, according to the 

 plane of the greatest extension of the bone. (3) The minute ex- 

 ternal covering of the osseous plates is formed by a delicate layer of 

 Osteodentine, which, in the dentigerous bones, continues directly into 

 the dentine cones of the small cuspidate teeth. 



The same homology exists between the minute palatine teeth of 

 Sclerocephalus and the small teeth of the Ganoid scales ; both are 

 cuspidate dentine cones, provided with a delicate enamel cap and a 

 large pulp ; their basal expansions are fused into the layer composed 

 of a modification of dentine (Ganoin), which covers the second 

 stratum above described. 



The histological structure of the large jaw teeth is closely investi- 

 gated on ten transverse sections, and leads the author to the con- 

 clusion that these plicated teeth (Faltenzdhne) of Sclerocephalus owe 

 their origin to the coalescence of the elements (Anlagen) of numerous 

 most primitive teeth, similar in shape to those on the palatine bones ; 

 whereas the cement-ridges ( Cementleisten) of the jawbones are formed 

 by a fusion of the basal layers of the jaw teeth. 



All the larger teeth, uni- or multi-cuspidate, of the Eotetrapoda, 

 and consequently likewise of the higher Vertebrates generally, are 

 thus shown to be poly synthetic, viz. produced by a fusion of the pulps 

 of numerous tooth-germs (Zahnanlagen), so that they do not represent 

 primitive, but phylogenetically acquired structui'es. 



1 H. Credner, Die Stegocephalen iind Saurier sus dem Eothliegenden des Plaiien- 

 schen Grundes bei Dresden, Part I-X (Zeitschr. d. Deut. geolog. Gesellschaft, 1881- 

 1893). — H. Credner, Die Urvieri'iissler (Eotetrapoda) des Sachsischen Eothliegenden. 

 Berlin, 1891, etc. 



