216 ORTHOGEXETIC VARIATION IN THE SHELLS OF CHELONIA. 



some scutes and plates increase steadily in size, while others remain comparatively 

 stationary, so that in the latter case they apparently undergo a reduction or even 

 suppression. 



Let us now arrange the various Chelonians, first Thalassochelys caretta, according 

 to the hints which we have received from the analytical examination of the specimens 

 catalogued above. 



Sta^e I. Hypothetical. Eight neural and eight pairs of costal scutes. Neurals 

 and costals to lie in the same transverse plane. 



Stage II. Eight neurals and eight pairs of costals, the latter fitting with then- 

 median or proximal ends in between two successive costals. This rearrangement seems 

 to have been brought about by the partial reduction of one pair of costal scutes. 

 This reduced scute is probably the 2nd costal. Cf. nos. 1 and 44. 



Stage III. Eight neurals and seven pairs of costals. The original 2nd costal 

 having been suppressed, the original 3rd has now become the actual 2nd and abuts 

 between the second and third neural, the rest following correspondingly 1 ; the actual 4th 

 (the original 5th) now lying between the 4th and 5th neural, i.e. half in front of the 

 latter to which it genetically belongs, as is still indicated by its evident sympathy 

 with this neural.— Nos. 2 and 3. (Text fig. Ill" and IIP.) 



Stage IV. Seven neurals and seven pairs of costals. Brought about by reduction 

 of the original 7th or last but one neural, the last neural has thereby become the 

 actual 7th in nos. 8 — 13, while nos. 2 and 3 represent an intermediate stage in the 

 condition of the 7th neural. 



Stage V. Seven neurals and seven pairs of costals, but the 5th neural and the 

 4th pair of costals are in a state of reduction. Cf. nos. 9, 10, 11, 12. 



Stage VI. Six neurals and six pairs of costals, owing to complete suppression of 

 the 5th neural and the 7th pair of costals. The original 6th neural is now number 

 5 and the last is now number 6, late 7, late 8. A similar rearrangement of the 

 serial numbering applies to the costals; the 3rd (late 4th of stage I.) and the late 

 5th having now become neighbours. Cf. nos. 28 — 42 with numerous instances inter- 

 mediate between the last two stages. (Text fig. VI a and VI b .) 



1 This change in the numbering of the scutes shows how difficult it is to arrive at the true homologies 

 of the individual parts which exhibit meristic variation. It will often be absolutely impossible to homologise 

 such parts unless we know the whole developmental history of the whole creature, or as in the present 

 case, unless we can study an ample material which provides almost every theoretically possible intermediate 

 stage, or which comprises certain individual variations which can be submitted to a crucial test. Arguing 

 by deduction we can then (e.g. specimens, numbers 7, 13, 17, 24, 43) check the correctness of the conclusions 

 which we have arrived at by the inductive method. 



In Thalassochelys the numerical changes are as follows : 



Starting with 8 neurals, suppression of the 7th neural turns the original 8th, or last, into the actual 

 7th, subsequent suppression of the oth turns the original 6th into the 5th and the 7th, late 8th, into the 

 6th neural. Ultimate loss of the 1st, e.g. in Testudo sumeirei changes the 2nd into the now actual 1st and 

 the last of all, the late 8th, late 7th, late 6th into the 5th. 



A similar rearrangement of numbers is necessary with the costals after the dropping out of the original 

 2nd, the 5th and one or two of the last pairs. 



In order to avoid confusion as much as possible, the true homologies of the scutes have been indicated 

 in the diagrams (p. 217) by corresponding shading. 



