Proteus . 



Siren 



Menopoma 



Lepidosiren 



Pterodactyle 



TEETH 1023 



Long Diameter Short Diameter 



1-570 to 1-980 1-885 to 1-1200 



1-290 ., 1-480 1-540 1-975 



1-450 1-700 1-1300 1-2100 



1-375 1-494 1-980 1-2200 



1-445 1-1185 1-4000 1-5225 ' 



In preparing sections of bone it is important to avoid the pene- 

 tration of the Canada balsam into the interior of the lacunae and 

 canaliculi, since when these are filled by it they become almost 

 invisible. Hence it is preferable mot to employ this cement at all, 

 except it may be in the first instance, but to rub down the section 

 beneath the finger, guarding its surface with a slice of cork or a slip 

 of gutta-percha, and to give it such a polish that it may be seen to 

 advantage even when mounted dry. As the polishing, however, 

 occupies much time, the benefit which is derived from covering the 

 surfaces of the specimen with Canada balsam may be obtained 

 without the injury resulting from the penetration of the balsam into 

 its interior, by adopting the following method. A quantity of 

 balsam proportioned to the size of the specimen is to be spread upon 

 a glass slip, and to be rendered stifFer by boiling, until it becomes 

 nearly solid when cold ; the same is to be done to the thin glass 

 cover ; next, the specimen being placed on the balsamed surface of 

 the slide, and being overlain by the balsamed cover, such a degree of 

 warmth is to be applied as will suffice to liquefy the balsam without 

 causing it to flow freely, and the glass cover is then to be quickly 

 pressed down, and the slide to be rapidly cooled, so as to give as 

 little time as possible for the penetration of the liquefied balsam into 

 the lacunar system. The same method may be employed in making 

 sections of teeth. 2 The study of the ossein or organic basis of bone 

 should be pursued by macerating a fresh bone in dilute nitre-hydro- 

 chloric acid, then steeping it for some time in pure water, and 

 tearing thin shreds from the residual substance, which will be 

 found to consist of an imperfectly fibrillated material, allied in its 

 essential constitution to the ' white fibrous ' tissue. 



Teeth. The intimate structure of the teeth in the several classes 

 and orders of Yertebrata presents differences which are no less 

 remarkable than those of their external form, arrangement, and suc- 

 cession. It will obviously be impossible here to do more than sketch 

 some of the most important of these varieties. The principal part of 

 the substance of all teeth is made up of a solid tissue that has been 

 appropriately termed dentine. In sharks as in many other fishes 

 the general structure of this dentine is extremely similar to 

 that of bone, the tooth being traversed by numerous canals, which 

 are continuous with the Haversian canals of the subjacent bone, and 

 receive blood-vessels from them (fig. 754), while each of these canals 



1 See Professor J. Quekett's memoir on this subject in the Trans. Microsc. Soc. 

 ser. i. vol. ii. ; and his more ample illustration of it, in the Illustrated Catalogue of 

 the Histological Collection in the Museum of the Royal College of Surgeons, 

 vol. ii. 



2 Some useful hints on the mode of making these preparations will be found in 

 the Quart. Journ Microsc. Sci. vol. vii. 1859, p. 258. 



