EEMARKABLE TORTOISE, TESTUCO LOVERIDGII. 519 



shield-sutures, and are thus adapted to support four instead of five 

 vertebrals. This fact points to dermal origin of the plates, but is 

 in opposition to Parker's theory of ectodermic migration in 

 Ohelonia {a propos of the tadpole's tail), in which he points out 

 that "any early local distui^bing influence that aflfected both scute- 

 and plate-producing tissues would leave its trace in the adult in 

 the form of a region of modified scutes posterior to a region of 

 modified bony plates." At the same time, this does not alter the 

 fact that the plates ai'e adherent to the bones of the true skeleton, 

 and are coincident with them regarding numbers etc. Yet 

 development seems to be influenced and regulated chiefly by that 

 of the overlying dermal areas corresponding to the epidermal 

 shields. This is also seen in the Giant Tortoises known to 

 have thin carapaces, such as T. abingdonii, microphyes, etc., in 

 T. calcaratus, and in fossil forms such as the Pleistocene Marsh 

 Tortoises, T. vosmaeri from Rodriguez and T. indica from 

 Mauritius. In these the form of the plates is similar to that of 

 T. loveridgii, the essential network beneath the shield- sutures being 

 of thick hone standing out in relief from the thin bone, tahich fills 

 in ivhat ivoidd be fenestrce in the young or in loveridgii. 



In ordinary adult tortoises in which the carapace is complete 

 and of equal thickness, this cannot be appreciated, but in these 

 it can be studied in the initial stages of plate-development. 



The following quotation from Owen (1849, p. 161), whose paper 

 I had not read at the time of my own observations, coiToborates 

 in every particular what I have found to be the case in so many 

 species : — 



" A strong argument for regarding the costal plates as dermal 

 ossifications rather than jDrocesses or continuations of the endo- 

 skeletal elements, to which they are attached, maybe drawn .... 

 from the period of their ossification, and their relative position 

 to the ribs with which they are connate. 



" In the embryo Testudo indica* the uniformly slender pleura- 

 pophyses are ossified nearly throughout their whole length 

 before the ossification of the costal plates, usually regarded 

 as their expanded tubercle, commences ; and the beginning of the 

 superadded bonet is not at the same point in each rib, as might 

 have been expected if it were the exogenous process called 

 ^ tubercle ' of the rib. The costal plates are situated in the 

 young Testudo indica alternately nearer to and farther from 

 the head of the rib; and their presence seems to be deter- 

 mined rather by the angle of union of the superincumbent 

 vertebral scutella with the lateral or costal scutella, than by 

 the necessity for additional strength of the articulation of 

 the ribs with the spine. Ossification commences at the point 

 from which the three impressions radiate, and as this point is 



* [Probably T. nigrita or elepliantina, indica Schn. being an extinct species.] 

 f " This period, in its relation to the development of the neural arches and pleura- 



pophyses, corresponds precisely with that at which the dermal plates of the Crocodile 



begin to be ossified." 



