A HUMP-BACKED TROUT FROM STRANRAER 225 



numbers of haemal arches in the caudal, and ribs in the 

 abdominal regions. For convenience, lest the use of the 

 term " centrum " should be misleading in so abnormal a 

 specimen, I shall call that portion of the backbone between 

 two divisions or nodes an internode. Further, since there 

 is meristic agreement between the dorsal neural spines and 

 the ventral spines and ribs, I shall refer simply to the 

 former, it being understood that such remarks, so far as 

 numbers and position are concerned, apply equally to the 

 ventral elements. 



Since, then, neural spines are more numerous than 

 internodes, it must happen that in many cases crowding 

 of the spines has taken place ; and, indeed, the bases of as 

 many as three sometimes rest on a single internode. The 

 distribution of the spines on the internodes is as follows : — 

 Reckoning from the anterior end of the vertebral column 

 the nodes which are normal vertebrae, bearing each a 

 single spine, are i to 4, 6, 7, 9 to 12, 14 to 17, 21, 24, 

 26, 28, 29, 31, 33, 36 to 40. Of the remainder, two spines 

 are borne by 5, 8, 13, 18, 19, 20, 23, 25, 32 and 34 ; three 

 spines by 22, 27, 30, and 35. The normal vertebrae call 

 for no remark. The other internodes, those on which two 

 or more neural spines are borne, are characterised in general 

 by being slightly longer than normal internodes. They 

 show no trace of segmentation, but, on the larger, hints of 

 lateral projections occur, corresponding in number and 

 position with the neural spines, and marking the places to 

 which the muscles were attached. 



It is also worthy of note that the muscle segments 

 correspond with the neural processes, with which also the 

 spinal nerves agree. The segmentation of the body, then, 

 is normal. The fin-rays too fall within the limits of varia- 

 tion recorded for the species, for the dorsal possesses 1 3 rays 

 (12-15),^ the pectorals 15 (13-15), the ventrals 9 (9), the 

 anal 10 (10-12), and the caudal 19 (19). So that 

 abnormality occurs in the vertebral column and there only. 



It is difficult to account for the origin of such abnormality. 

 In the only paper which I have seen referring to a similar 



^ The numbers in brackets show the variability in fin-rays as given by 

 Day, I.e. 



6Z D 



