ORTHOGEXETIC VARIATION" IX THE SHELLS OF CHELOXIA. 



213 



The following table contains the variations or combinations in the number of scutes 

 which can possibly occur. Those which have actually been observed are indicated by 

 black type. The whole number of possibilities amounts to 30, and it seems only a 

 question of greater material than has been at my disposal to find the hitherto missing 

 variations. From the morphological point of view it is of course immaterial if the 

 right or the left side contains the greater number of costals. 



The chief abnormalities are distributed as follows. 



With eight neural scutes: eight new-born, and two adults. 



With seven neural scutes: 13 new-born, one of 13, and two of 8 inches. 



With mx neural scutes, but with abnormal costals: 23 new-born and two young (of 

 4 and 8 inch< - 1. 



Of these 51 abnormal specimens 26 are serious in so far as they refer to specimens 

 with seven or eight neural scutes, while only two of these specimens (numbers 18 and 

 19) have normal costals. Moreover of these 26, only five are not baby specimens. 



Of the remaining 25 abnormal specimens only two are not babies. Not less than 

 15 of them are quite symmetrical with the formula 6, 6, 6 ; 4 are asymmetrical by 

 possessing one extra right and two extra left costal scutes or vice versa, and four are 

 symmetrical, possessing seven instead of five pairs of costal scutes. 



In every case in which the last but one median scute is very small or vestigial 

 it is the 7th scute: in other words the last but one neural occurs in a vestigial 

 condition only in specimens with the large number of eight neurals. This applies even 

 to the two specimens Nos. 13 and 17 in which there are now only seven neurals, but 

 which by deduction can be shown to have possessed the original full number of eight 

 neurals. Moreover in none of those specimens with a vestigial last but one neural 

 (Nos. 1, 2, 3, 7, 13, 17, 196) are any of the corresponding 6th or 7th pairs of costals 

 exceptionally small. It seems reasonable to suppose that, whilst the last but one 

 neural dwindles away, the last pair of costal scutes increases in size. An indication 

 of this process is the frequently crowded position of the proximal ends of the two 

 last pairs of costal scutes. 



In a considerable number of specimens (17 new-born specimens and one 4 inches long) 

 the 4th costal scute is abnormally small, and this reduction coincides in eight cases with 

 a half-suppressed condition of the 5th neural. It is significant that seven of the 

 specimens with such a reduced 5th neural still possess either eight or seven neurals, 



