THE DENTAL TISSUES OF THE ORDER RODENTIA. 
533 
fish we have the teeth of the Dendroclus, the Lepidosteiis, Myliobates, and sharks of 
the g’enns Lanina, also composed of a series of dentinal systems. In all of these the 
open extremities of the dentinal tubes are closed previous to the part being exposed 
by wear. A similar condition, in a beautiful state of preservation, is found in many 
fossil fish teeth, especially in the Cestracions. I cannot refrain from noticing one 
other instance of this condition. The antlers of the Stag are composed of Flaversian 
systems of medullary or vascular canals, surrounded by concentric laminae of osseous 
tissue interspersed with lacunae, the canaiiculi of which anastomose freely, and those 
situated near a vascular canal terminate by open mouths on its surface. Previous 
to the shedding of the antler, each of the larger canals becomes lined v/ith a layer of 
transparent, dense and almost structureless tissue, which completely closes the mouths 
of the canaiiculi and cuts olf the conneetion of the elaborate system of tubes and la- 
cunae with the vascular canals (fig. 4). 
The division of the enamel of the incisors into two layers, described by Professor 
Owen, I have found common throughout the order, excepting in the incisors of the 
Hares and the Rabbit ; the Lagomys I have not had an opportunity of examining, 
but from their close relation to the Hares, it is more than probable that in their in- 
cisors the enamel is not divided into an outer and inner layer. 
The term layer is open to objection, as the two parts are made up of continuous 
fibres. In the inner part they decussate, while in the outer they are parallel, but their 
continuity may be distinctly traced (fig. 6 to 50). In the molars of many Hystricine 
teeth, the usual order is reversed ; in the inner portion the fibres are parallel, and in 
the outer part of the enamel they decussate. 
Professor Owen, in the passage cited, and in other parts of his great and valuable 
work, states that the cementum is continued over the enamel in the incisors of ro- 
dents, and objeets to some of Professor Erdl’s figures printed in his work on the 
Microscopic Structure of the Molars of Rodentia, because this tissue is left out. He 
says Professor Retzius failed to recognize the cementum from its being coloured. 
Professor Owen*, in his description of the incisor of the Water Vole, says, “ The layer 
of cement becomes thinner at the margin of the enamel, where it is continued from 
the dentine upon that part, but soon inereases in thickness, acquiring the bright brown 
tint, and separated by a well-defined line from the outer clear layer of the enamel.” 
I have sought with care for cementum on the anterior surface of the incisors of 
the Water Vole, of which the foregoing quotation is a description, and also in num- 
berless other teeth, but have failed to find that tissue. In most, if not in all incisors 
of rodents, cementum may be seen investing the posterior surface, and it may be traced 
dentine, much in the same manner as the confluent denticles of the molar teeth of Rodentia are united. In the 
teeth of the Lepidosteiis and Lizards of the genus Varanus, the dentine of the middle and basal portions is divided 
into systems somewhat in the same manner as in the Labyrinthodon, and in the upper part of the tooth within 
a general circumferential investment of enamel. I have in m3’' possession sections from two species of Varanus, 
V. Bella and V. Niloticus, in which this point is incontrovertibly shown. I am indebted to the kindness of 
Dr. Andrew Smith for the teeth of these species. * Odontography, page 405. 
