HADROSAURC S. 



89 



of the teeth, and is to be viewed as the remains of the pulp cavity. It is pervaded 

 with vascular canals, the cut orifices of which may be seen in the section. 



The dentinal tubules pursue a gently undulating nearly parallel course, and the 

 successive waves in the section, when viewed by transmitted light, appear alternately 

 darker and lighter, giving rise to the impression of a concentric laminar arrange- 

 ment of the dentinal structure. The tubules are exceedingly fine and arrano'ed 

 very closely together. They branch in their course outwardly and measure from 

 the jgJooth to the aoo^oo^li of an inch. 



The inner surface of the section (Fig. 1, «) is bordered by an enamel layer about 

 the Jjjth of an inch thick. The enamel folds around the lateral borders of the 

 crown and ceases in a rather abrupt but thin edge. In the drawing, there is repre- 

 sented a section (b) of a narrow isolated streak of enamel, which extended a short 

 distance along the side of the crown. 



The outer border of the section (Fig. 1, d, Fig. 2, b) presents a pecuhar layer, 

 about ^\yth of an inch thick, apparently consisting of a spongy reticulation of 

 vascular tissue or vaso-dentine. It is this layer which gives the surface of the 

 teeth, Avhere not covered with enamel, a dull aspect ; and it is friable and easily 

 scraped off from the denser dentine within. The vascular canals of the spongy 

 tissue are of nearly uniform size and measure about the loVo*^ of an inch in 

 diameter. From the vaso-dentine many of the vascular canals penetrate into the 

 denser dentine, in some cases to a greater depth than the whole thickness of the' 

 former, as represented in Fig. 2, Plate XX. 



The specimens of bones of the anterior extremity of Hadrosaurus consist of the 

 humerus and those of the forearm of the left side. 



The humerus of Hadrosaurus, represented in Figs. 1, 2, 3, Plate XIV, bears a 

 near resemblance in form and proportions to that of the Iguanodon as figured by 

 Dr. MantelP and Prof. Owen.^ The upper part of the shaft is twice the breadth 

 of its thickness, is concave transversely in front, and convex behind in the same 

 direction, and has its inner border thinner than the outer. The lower part of the 

 shaft is cylindroid, and gradually expands towards the condyles. 



The bone is broadest at its upper extremity from which the head projects pos- 

 teriorly midway between two nearly equal tuberosities, which extend almost as 

 high as the former. The head (a) is a hemispherical roughened prominence partly 

 sustained by an abutment-like ridge extending doAvnward on the back of the shaft. 

 The upper part of the inner tuberosity (h) is convex and rough. The upper part 

 of the outer tuberosity (c) is sigmoid and liKewise rough. 



From the inner tuberosity, the internal border of the shaft makes a continuous 

 concave sweep to the lower end of the bone. From the outer tuberosity, the shaft 

 remains expanded externally, to accommodate the attachment of the deltoid muscle, 

 nearly to its middle (d), when the bone rapidly narrows to its lower cylindroid por- 



* Phil Trans. Roy. Soc, Lond., 1849, Plate XXXi. Petrifactions and their Teachings, p. 286, 

 Pig. 60. 



" British Fossil Reptiles, Dinosauria, Plate XIX, Figs. 3-6. 

 12 April, 1865. 



