CRYl'TODIKA. 13 



the median line. In a specimen of Clnostcrnmn aIho(/ulare, the first 

 neural plato is also absent. The shape of the neural plates aftV)rds 

 g'ood generic characters. They may be tetra-, hexa-, or octagonal ; 

 they may be all similar in shape (hexagonal), or hexagonal plates 

 may alternate with octagonal, or tetragonal >vilh octagonal (e. g. 

 Testudo). When hexagonal, their lateral sides may be of equal or 

 subequal length (c </. some Chelonida)), or the antero-lateral (o. g. 

 Emijs) or the postero-lateral (o. g. Cijclcmi/s) the shorter ; the hinder- 

 most plates are as a general rule short-sided in front ; the first is, 

 with few exceptions, tetragonal. It not unfretpiently happens for 

 one or more of the neural plates to split up into two or three, or for 

 two to amalgamate into one, but such are usually easily recognized 

 as anomalies. 



The costal plates number eight on eacli side*. In the newly- 

 hatched young they are scarcely developed, and the ribs do not come 

 into contact with one another ; ossification starts a short distance 

 from the neural plate and soon works up towards the latter ; but it 

 takes some considerable time until it reaches the distal extremity, a 

 stage which is attained but late in life in GhcJi/dra and Macro- 

 clemmys, and very late or perhaps never in the marine Turtles. In 

 B((i(i(/}ir smoll fontanelles persist between the nuirginals and costals 

 in specimens which have nearlj* attained full size. As a rule, the 

 carapace of land Tortoises ossifies more rapidly than that of fresh- 

 water. The distal extremity of the rib jjcrsists as a free point 

 fitting into a corresponding socket in tho marginal plate. The first 

 costal jjlate is constantly the broadest (pcr])endicularly to the axis), 

 the last the smallest ; in Cinosternum the latter is unusually small 

 and destitute of the distal point. In Tistudo, IJomopus, and Ciiiiri/s, 

 and to a lesser degree in a few freshwater Tortoises, especially old 

 individuals, these plates are unequal in width, alternately widening 

 proximally and narrowing distally. 



Continuing the sei'ies of the neural plates are two to four azygous 

 membrane bones termed the pygal plates, the last of which, in sha])o 

 and position, forms part of the marginal series; this distinction is 

 most obvious in immature Chelonida;, the azygous marginal being 

 separated from the true pygals by an interspace. 



In ThaJassochdi/s usually and in CJielonc exceptionally, the last 

 pair of costals meets on the middle line, between the last neural and 

 the anterior pygal, of which two or three arc present besides the 

 azygous marginal. 



In addition to the azygous posterior marginal, there are eleven 

 (exceptionally twelve) marginal plates on each side, except in 

 Stauroti/jnis and Cinosternum, which have only ten. 



Plastkox. — The plastron is composed of nine bones t, viz. an 



* Nine or ten in some fossil marine forms. 



t In the fossil Baenidse an additional bone {iiiesoplastron) is present on each 

 side, between the hyo- and the hjpoplastron. 



