Januaby 1, 1890.1 



KNOWLEDGE 



Tortoises and turtles, or, as it is frequently convenient 

 to call them, Clielonians (from the Greek name of the 

 turtle), are, however, in reality a very remarkable group, 

 or order, of the great class of Reptiles ; and their form 

 is so peculiar and so interesting that a short glance at 

 some of their chief structural features cannot fail to be 

 instructive. We are, indeed, accustomed to regard many 

 extinct groups of reptiles, like the Fish-lizards '■' of the 

 Secondary rocks, as more hi^driv and strange than any 

 forms which now mhabit the globe ; but if we were to be 

 made acquamted for the first time with tortoises from their 

 fossil remains, we should certainly consider them as far 

 more extraordinary than any other types ; and it is highly 

 probable that the palieontologist who first made known 

 such a remarkable type of reptilian structure would be 

 charged with having created a totally impossible monster. 



The most striking and peculiar feature about tortoises 

 and turtles is the more or less complete bony shell with 

 which their body is protected ; and on accomit of wliich 

 they are noticed in the article on "Mail-clad Animals" 

 published in the December number of Knowledge. The 

 accompanying woodcut (Fig. 1) exliibits a typical Chelo- 

 nian, as exemplified by a land-tortoise. In this creature 

 we see a fully developi'd bony shell, within which the head, 

 limbs, and tail can be retracted, so as to afford a j)erfect 

 protection for the entire animal. We have just said that 

 the shell of the tortoise is a bony one ; but it will probably 

 be at once objected that the " tortoise-shell" of commerce 

 is about as unlike bone as it can well be. This apparent 

 discrepancy can, however, be very readily explained. In a 

 living tortoise, as is well shown in Fig. 1, the outer 

 surface of the shell is completely covered over with a series 

 of large shield-like horny plates, of which there is one row 

 down the middle of the back, a lateral row on either 

 side of this middle one ; externally to which we have a 

 series of marginal shields, forming the border of the upper 

 half of the shell. Similar horny shields also cover the 

 lower part of the shell, which we shall notice shortly. 

 The solid bony shell imderlies these horny shields ; the 

 relations of the horny and bony constituents of the protect- 

 ing shell being shown in Fig. 2, where the outer horny 

 shields have been strijiped off, leaving distinct grooves on 



the beautiful colouration formed by the mottled blending 

 of a full reddish bro-i\-n with a lemon yellow. It will be 

 noticed fi-om Fig. 1 that in the land-tortoises these 

 horny shields have their edges in apposition ; but in the 



Fis. 3, — The Middle and 

 Left Side of the Loweb 

 SHRr.L, OR Plastron, of 

 A Land-Tortoise. 



Fig. 2.— The Right Half 

 OF THE Upper Shell, or 

 Carapace, of a Tortoise, 

 with the Horny Shields 

 REMOVED. The thick block 

 lines show the boundaries 

 of the horny shields, while 

 the zigzag lines indicate the 

 divisions between the under- 

 lying bonv elements of the 

 shell. 



young of the Hawksbill turtle they overlap one another 

 like the slates on a roof, although in the adult they 

 become united by their edges in the same manner as in 

 the tortoises. 



1. — Sill]; Vie 



1F A Land-Tortoise, with the Heali and Limh.s 



IKOTKliDED FROM THE SlIELL. 



the underlying bones at the lines of junction with one 

 another. Now it is these horny shields which form the 

 "tortoise-shell" of commerce ; in the land-tortoises they 

 are, indeed, very thin, and of uo economical value, but in 

 one of the nuirine turtles, known as the Hawksbill, they 

 become greatly thickened, and furnish the well-known 

 " tortoise-shell," so remarkable for its translucency and 



See Know i.EiK^K for Xovember 1889. 



Turning once more to Fig. 1, it will be 

 seen, as we have already incidentally 

 mentioned, that the shell consists of an 

 upper vaulted portion covering the sides 

 and back, which is teclmically known as 

 the r((i-(ijiacc ; and also of a flattened 

 lower plate protecting the chest and 

 ;ibdomen, to which the term plastron, or 

 lireast-plate, is applied. The carapace 

 and the plastron are usually connected 

 together by a comparatively short bony 

 bridge, at the extremities of which are 

 the notches for the fore and hind limbs, 

 as is well shown in Fig. 1. In the land- 

 tortoises this union between the carapace 

 and plastron is quite complete ; but in 

 the marine-turtles, and also in some freshwater-tortoises, 

 there is no bony union between the upper and lower por- 

 tions of the shell. The characters of the plastron of the 

 land-tortoises are well shown in Fig. 3, from which it will 

 be seen that there are six pairs of symmetrical horny 

 shields, arranged on either side of the middle line of the 

 shell. This type of plastron is characteristic of nearly all 

 the tortoises of the northern hemisphere ; but in a smaller 

 group, now conlined to the southern hemisphere, there is 

 an additional unpaired shield (Fig. 4, /, gii) dividing the 



