chap, in.] vertebrae: reptilta. 123 



above described ; but in 4 F. domestica, and 2 F. catus in Edinburgh, 



I F. domestica, 1 F. catus, 1 F. concolor and 1 Cynozlurus jiibattis 

 in Cambridge the change is between the 17th and 18th. For informa- 

 tion as to the Edinburgh specimens, I am indebted to Professor 

 Struthers.] 



50. Galictis vittata. Specimen from Parana had 16 pairs of ribs, 



I I true and 5 false ; 5 lumbar, 2 sacral and 2 1 caudal vertebra?. 



A specimen from Brazil had only 15 pairs of ribs and the same 

 number of lumbar and sacral vertebrae. Burmeister, Reise durch d. La 

 Plata-Staaten, Halle, 1861, n. p. 409. 



[This is therefore another case of forward Homoeosis, (as manifested 

 in the presence of an additional pair of ribs) associated with an increase 

 in the number of presacral vertebra?.] 



51. Halichcerus grypus. Phocida? generally have C 7, D 15, L 5. 

 Specimen of //. grypus having C 7, D 15, L 6 at Berlin. The an- 

 terior of the six lumbars bears a rudimentary rib about 5 cm. in length 

 on the left side. The 28th vertebra is here detached from the sacrum 

 giving S 3, but generally it is united to it, giving S 4. Nehring, A., 

 Sitzb. naturf. Fr. Berlin, 1883, pp. 121 and 122. There is here 

 therefore a forward Homoeosis in the development of a rib on the 

 23rd, and also in the detachment of the 28th from the sacrum. 



Reptilia. 



52, Mr Boulenger kindly informs me that though the number of 

 ventral shields (which is the same as that of the vertebra?) is as a 

 rule very variable in the several species of Snakes as a whole, there 

 is nevertheless great difference in the degree of variability. A case 

 of maximum variation is that of Polyodontophis SUbpunctatus, in 

 which the number of ventral shields has been observed to vary from 

 151 to 240 (Boulenger , Fauna of Brit. India; Reptilia etc. 181)0, 

 p. 303). 



53. On the other hand the range of variation in Tropidonotus natrix 

 is unusually small. Among 141 specimens examined the number of 

 ventral shields varied from 162 to 190 (Strauch, Mem. Ac. Sci. Pet., 

 1873, xxi., No. 4, pp. 142 and 144). 



*54. Gavialis gangeticus. In this animal there are normally present 

 24 presacral vertebra? and 2 sacrals, the first caudal being the 27th. 

 This vertebra has a peculiar form, being biconvex. Specimen de- 

 scribed having 25 pnesacrals, 2 sacrals, the 28th being the first 

 caudal. Baur, G., J. of Morph., iv., 1891, p. 334. In this case 

 Baur argues that since the first caudal is clearly recognizable by 

 its peculiar shape, this vertebra must be " homologous " in the 

 two specimens and he considers that a vertebra must have been 

 " intercalated " at some point anterior to the first caudal by a process 

 similar to that seen in Python (see No. 7). In his judgment this 

 has occurred between the 9th and 10th vertebra?, but no reason for 

 this view is given. On the system here adopted, this would be spoken 

 of as a case of forward Homoeosis. 

 55. Heloderma. The first caudal in the normal form may be dis- 

 tinguished by having a perforation in the small rib connected with 



