396 M ERISTIC VARIATION. [PART I. 



of the development of the limbs and digits, ranging from C. ocellatu* 

 and '. lin/i-iiii/ii' with pentadactyle limbs fairly developed, through 

 C. lineatus (tridartyl.-) and C. tridactylus to C. gueniheri in which 

 the limbs are minutf conical rudiments. Amongst the species of 

 this serie- great individual variations occur. 



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

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



ha- OB > ;ieh hind tout five digits. 



008. C. sepoides : Mr BOULEXGER tells me that the normal num- 

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

 four digit- mi each t<>ot. 



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

 t ui-tlcs as defined by AGASSIZ (X. Amer. Testudinata, (.'<>ntrib. to 

 X. 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., 

 IM-'.t. p. 10) 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 Hrlt. Mns. 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. 



AGASSI/ in is"; 7 (I.e.) divided Cistudo into four species, giving 

 to the Mexican form the name C. triniif/td.^, 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 Onychotria cannot stand." The form found from New 

 Kngland to the Carolinas is called by Agassiz C.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. ,x. p. 65) stated that the three- 

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

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

 foot and four on the other. 



010. Rissa 1 . The commoD Kittiwake (R. tridactyla) as found in 



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

 reference in:iy In- made to the fact that among birds there are several examples of 

 >]'<<!< s differing from their near allies by reason of the absence of the hallux. 

 Speaking of this feature in Jocamaralcyon triilni-ti/hi, SCLATER observes: "In the 

 pn -rnt bird we meet with :iin>tlnT example of the same character [viz. a monotypie 

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

 above mentioned, Judimtir<iti->/<i being notably different from all other members 

 of the (ialbnlidne 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 



