209 



panded tubatlaa, BoaiBSJSMns t and the beginning of the mprrmdded bone is not at the same 

 |N>int in each rib, as might have been expected if U were the exogeaoM process called 'tubercle' 

 of the rib. The costal platM are situated alternately nearer to and farther from tin- hrad rf 

 the rib ; and their presence a0enu to be deterainad rather by the anglr of union of thr i|tcr- 

 inriimlx-iit vertebral acuteUa with the lateral or costal scutella, than by the necessity for 

 niUlitional strength in the articulation of the rib* with the spine. OMJicKion commences at 

 thr point from which the three impressions radiate, and at this point is nearer thr median 

 line at the median apex of the costal scuteUum than at the lateral apex of the vertebral scu- 

 trllum, the resulting plates of bone are alternately further from or nearer to the middle linr . 

 and the first, third and fifth costal plates hare adranced along the proximal end of the rib o 

 as to join thr neural plates, whilst the second, fourth and sixth costal plates Icare a portion 

 of the proximal end of the rib uncovered and crossing the space between the incipient costal 

 plate and the neural plate. In regarding these incipient ossifications, extending into the sub- 

 stance of the cerium and receiving the impressions of the epidermal scutes, as the developed 

 tubercle ' of the ribs, as Rathke has rndeavoiired to illustrate in tab. 3. figs. 1 1 (Tortoise), 

 12 and 13 (Chick) of bis elaborate Monograph *, we are compelled to suppose that each 

 succeasiTe rib in the Tortoise has a different position of its tubercle, which is alternately 

 Mr and farther from the head, and that the neck of each successive rib is alternately long 

 and short, which is contrary to all analogy furnished by those cold-blooded or warm-blooded 

 I'rrtebrata that have unquestionably the exogenous process called ' tubercle ' dereloped from 

 the true neck of the rib. 



There is an obvious difference in the texture and external surface of the bones which un- 

 >ly belong to the endo-skeletal vertebra;, and of those which, notwithstanding their 

 i with the neural spines and pleurapophyses, are developed in the fibrous substance 

 of the cerium. These nascent ' neural ' and ' costal ' plates of the carapace have a granular 

 exterior and a coarse spongy texture, whilst the neural arches and pleurapophyses are com- 

 pact, smooth, and with a polished external surface : the part of the pleurapophysis which 

 passes beneath and is attached to the under surface of the costal ' plate contrasts strongly 

 with that superimposed dermal ossification. 



The marginal plates present the same rough, coarse, granular character as the neural and 

 costal plates : they are in no way connected in their development with the pleurapophyses, 

 which do not yet reach them : their ossification has been governed by the presence of the 

 marginal epidermal scutes, and, as in the case of the costal plates, by the points of junction 

 of contiguous scutes ; each marginal ossification is accordingly impressed by the lines indi- 

 cating the junction of the marginal epidermal scutes with each other and, in the case of the 

 middle ones, with the contiguous scutes of the plastron. The number of the marginal plate* 

 accords, moreover, with that of the marginal epidermal scutella, not with that of the ribs. 



M*. Brit. 

 1098. The plastron of the same immature Tortoise. 



It presents the sane difference in the texture and surface of the endo-skeletal and exo- 



* Ueber die Entwickelung der Schildkroten, 4to. 



ll 



