238 REPORT— 1873. 



Sclerotic Orbital Ring. — In Arcliegoscmrus Decheni* and A. latirostrisf, a 

 series of ossicles, which undoubtedly constituted a bony sclerotic ring, has 

 been found. As many as twenty-three ossicles have been observed in one 

 specimen ; but, owing to their scattered position and the perishable natui'e of 

 the contiguous parts, no example shows the series in its true position. The 

 annular arrangement is distinctly visible in one specimen. The individual 

 ossicles are of nearly quadrilateral form J. 



Teeth. — Tt appears from the observations of Von Meyer that the tooth of a 

 Labyrinthodont (ArcJiegosaurus) consists at first of a minute hollow cone of 

 enamel armed with two vertical diametrically opposite ridges. This, the true 

 crown of the tooth, retains its original structure and size until it disappears 

 by abrasion or fracture §. It does not, however, remain in its original 

 position, sessile upon the alveolar surface, but is gradually elevated upon a 

 conical base. This base, which is often the only part of the tooth preserved, 

 has the general form of a hollow cone of dentine, coated thinly with enamel, 

 and enclosing a pulp-cavity. The dentinal wall in a well-characterized 

 Labyrinthodont becomes folded longitudinally ; and some or nearly aU of the 

 folds may be again plaited. In a much convoluted tooth the folds are very 

 compact, and leave only linear spaces between them. In this way the thick- 

 ness of the dentinal wall is greatly increased, and the central cavity much 

 encroached upon. 



In Lahynnthodon, Prof. Owen describes a layer of cement as penetrating 

 such of the interspaces between the dentinal folds as communicate with the 

 exterior ||. This structure is certainly wanting in the Carboniferous Laby- 

 rinthodonts, where neither enamel nor cement is present between the folds of 

 dentine. A cross section of such a tooth as has been described exhibits a set 

 of sinuous and, it may be, branched interspaces communicating with the ex- 

 terior, and corresponding series (separated from the other by the dentinal 

 wall) of sinuous processes of the pulp-cavity. 



In some of the Carboniferous species there are no secondary dentinal folds ; 

 and it would appear from the descriptions that in some of the " Microsauria " 

 of Dr. Dawson the dentine is not folded at all. Externally the tooth is 

 grooved, and sometimes ridged also. It is frequently compressed in the 

 direction of the axis of the jaw, so as to present an oval or elliptical section. 

 Vertical edges (anterior and posterior) extending downwards upon the basal 

 portion are found in Loxomma. In Pterojjlax they are confined to the apex, 

 but are larger than usual. As a rule they are minute and not persistent. 



The teeth were attached to shallow depressions, which take the form of the 

 base and are often marked by radiating ridges corresponding with the den- 

 tinal folds. The mandibular alveolus is generally bordered by an external 

 ridge, which may be as much as a quarter of an inch high. 



There is always a premaxillary series, and, except where the maxilla is 

 wanting, a maxillary series also. The maxillary teeth may form an unin- 

 terrupted row ; or large tusks and depressious may occur at intervals. The 

 vomer and palatal are always dentigerous, giving attachment to an inner 

 longitudinal series, parallel with the outer or maxillary series. In Batra- 



* Goldfuss, Beitriige, p. 7, t. .3. figs. 1, 2 ; Von Meyer, 'Eeptilien' &c., p. 21, t. vi. 



t Ibid. p. 125. 



\ A sclerotic ring is present in Lacertilica, Chelonia, Ichthyosaiiria, Pterosauria, and 

 Birds, absent in all existing Fishes and Amphibia, Plesiosauria, Crocodilia, and Ophidia. 



§ In Loxomma the crown of the tooth is of great size, and extends far down upon the base. 



II Trans. Geol. Soc. vol. vi. 2nd series, p. 507, and 'Odontography,' pp. 201, 203. 

 There is no mention of inflection of the cjiamel, which, it is stated, "ceases at the base of 

 the crown." 



