84 H. H. NEWMAN. 



pairs of costals. These, if numbered from anterior to posterior, 

 would give the numbers I, 3, 5, 7, 9 and 1 1, to surviving neurals, 

 and numbers 2, 4, 6, 8 and 10 to vestigial or lost neurals ; the 

 numbers 3, 5, 7 and 9 to surviving costals, and numbers i, 2, 4, 

 6, 8 and 10 to vestigial or lost costals. In the tabulation these 

 numbers will be used without further explanation. Furthermore, 

 the sex, length and breadth of carapace, brief descriptions of both 

 scutes and bony plates, will be given in separate columns. The 

 significance of the tabulation of conditions of bony plates will be 

 seen later when the subject of correlation between scute and plate 

 abnormalities is discussed. The specimens are numbered and 

 arranged in the order of abnormality, the specimens with largest 

 number of extra scutes coming first, and those with less than the 

 normal number of scutes last. Extra neurals will be listed be- 

 fore extra costals and the latter before extra marginals. 



Two kinds of abnormality may be distinguished : symmetrical 

 and asymmetrical. The former are less common and are impor- 

 tant in that they furnish clearer cases and thus throw light on 

 the latter. Under the head of symmetrical abnormalities may be 

 mentioned extra neurals in the median line or nearly so ; extra 

 costals in pairs symmetrically placed ; extra paired marginals. 

 The great majority of abnormalities are asymmetrical, consisting 

 of: extra neurals crowded to one side or the other but usually 

 showing clearly enough the position they would normally oc- 

 cupy ; unpaired costals or marginals. In the case of asymmet- 

 rical neurals it is sometimes difficult to distinguish the supernu- 

 merary scute from the normal scute, on account of the large size 

 of the former and the fact that crowding has forced the two 

 scutes to lie approximately side by side. There are usually cor- 

 related points of asymmetry that may be of assistance in deciding 

 the point, but occasionally I have been compelled to trust to my 

 judgment and may possibly have erred. Gadow would probably 

 consider the type in which the normal and supernumerary scutes 

 lie side by side as evidence of the original paired character of the 

 neural row. Were it not for transitional conditions this view 

 might be tenable. 



Occasionally it becomes difficult to determine which of five 

 costals is the supernumerary scute, but a reference to the mar- 



