396 MERISTIC VARIATION. [part i. 



of the development of the limbs and digits, ranging from C. ocellatus 

 and C. bedriagce with pentadactyle limbs fairly developed, through 

 C. lineatus (tridactyle) and 0. tridactylus to C. guentheri in which 

 the limbs are minute conical rudiments. Amongst the species of 

 this series great individual variations occur. 



607. Chalcides mionecton : normally four digits on each foot. 

 A specimen in Brit. Mus. kindly shewn to me by Mr Botjlenger 

 has on each hind foot five digits. 



608. C. sepoides : Mr Botjlenger tells me that the normal num- 

 ber of digits on each foot is five, but that specimens occur having 

 four digits on each foot. 



609. Cistudo. This genus includes the North American Box- 

 turtles as defined by Agassiz (N. Amer. Testudinata, Gontrib. to 

 N. H. of U. S., I. p. 444). These animals are widely distributed 

 to the E. of Rocky Mountains. On the hind feet of some of them 

 there are three digits, while others have four. Gray (P. Z. S., 

 1849, p. 16) described two Mexican specimens which agreed in 

 having three large claws on the hind foot with no appearance of a 

 fourth claw, and even scarcely any rudiment of the fourth toe, 

 which was then believed to be present in the other members of 

 the genus. To this three-toed form he gave the generic name 

 Onychotria, but in Brit. Mus. Cat, 1855, he gave up this name as 

 a generic distinction, describing the Mexican form as Cistudo mexi- 

 cana, giving three toes on the hind foot as a definite character. 



Agassiz in 1857 (I.e.) divided Cistudo into four species, giving 

 to the Mexican form the name C. triunguis, and he states that the 

 western and south-western type is remarkable for having almost 

 universally only three toes on the hind feet. The toe which is 

 missing is the outer toe and " it fades away so gradually that the 

 genus Onycliotria cannot stand." The form found from New 

 England to the Carolinas is called by Agassiz G. virginea = C. Caro- 

 lina, and he states that he received a three-toed specimen from N. 

 Carolina which agreed in all other respects with those from New 

 England. 



Putnam (Proc. Boston, N. H. S., x. p. 65) stated that the three- 

 toed form found in the South is only a variety of C. virginea, and 

 that he had seen two specimens which had three toes on one hind 

 foot and four on the other. 



610. Rissa 1 . The common Kittiwake (R. tridactyla) as found in 



1 In illustration of the possible bearing of these facts on the problem of Species 

 reference may be made to the fact that among birds there are several examples of 

 species differing from their near allies by reason of the absence of the hallux. 

 Speaking of this feature in Jacamaraleyon tridactyla, Sclater observes: "In the 

 present bird we meet with another example of the same character [viz. a monotypic 

 form], and with one, perhaps, more isolated in its structure than any of those 

 above mentioned, Jacamaraleyon being notably different from all other members 

 of the Galbulidae in the absence of the hallux. At the same time we must be 

 careful not to put too high a value upon this at first sight seemingly important 



