250 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



In certain specimens from Vilamil (T. guntheri) one notes a 

 curiously pitted surface on some of the plates, as though they 

 were diseased. It is due to unequal shedding of the layers 

 of the horny plates. Why it should be confined to this region, 

 I cannot explain. Differences in color are very slight, but one 

 specimen shows on one plate a single diagonal clear yellow 

 ray, probably due to the absence of pigment cells at one point 

 of growth. 



When one sees side by side tortoises from several islands, the 

 differences which are most evident are those in the shape of 

 the shell. But, while these differences are noticeable and real, 

 they are subject to so much variation that their formulation is 

 most difficult, not to say confusing. In order to avoid hopeless 

 indefiniteness it is necessary to devise some means of express- 

 ing and comparing upon paper these variations in shape. It 

 was found that this could best be done by taking numerous 

 measurements of each tortoise and reducing all these measure- 

 ments to percentages of the (straight) length of the tortoise 

 In this way, the measurements of tortoises of all sizes may be 

 directly compared. The tortoise is placed upon a level board 

 or table in such a position that as nearly as possible it rests 

 naturally upon the entire length of the plastral bridge of each 

 side. With the tortoise in this position, the straight length 

 is the distance between verticals erected at the nuchal notch 

 and at the posterior border of the supracaudal plate. The 

 straight width is the distance between verticals erected at the 

 sides of the tortoise opposite the line of meeting of the second 

 and third costal plates. The curved length is measured with 

 a tape-measure over the midvertebral line from the nuchal 

 notch to the posterior edge of the supracaudal plate. The 

 curved width is taken from the bend in the marginal plates 

 up along the line of meeting of the second and third costals, 

 across the middle of the third vertebral, down between the 

 second and third costals, to the line of bending of the margi- 

 nals. The width second-to-third-marginals is the straight 

 width at the level of the lateral margins of the sutures between 

 the second and third marginal plates of each side. The middle 

 height is the vertical distance between the board or table and 

 the middle of the third vertebral plate, and is taken with a 

 square and spirit-level. The front height is taken in the 



