APPENDIXE: HEINCKE _ 14 _ 



form such favourable characters for the age-determation as in the plaice, owing to their 

 irregular lines or their relatively greater thickness. The separation of the 0- and I-groups 

 can be made here also, almost always without great difficulty, but the older stages are 

 often very difficult to distinguish or not at all, by means of the otoliths. It was desirable 

 therefore, to discover, if possible, still other anatomical signs of the age. First of all, the 

 scales had to be thought of, as undoubted age rings had long ago been discovered in 

 them, e.g. in the carp. Recently also, J. Stuart Thomson had closely studied, with 

 some success, the age-layers on the scales of sea-fishes. We have also examined the scales 

 and found them useful for the determination of the age in a few fishes, e. g. in the lemon 

 dab [Pleuronectes microcephalus). 



We have soon discarded the scales, however, as we discovered other organs more 

 suitable. An extensive and thorough investigation of all parts of the bony skeleton showed 

 us the noteworthy fact, that the growth in length, breadth and thickness of all bones of 

 the skeleton, is periodic and that the longest of these growth-periods, viz. the year, was 

 constantly marked in special layers, though with very different degrees of clearness in the 

 different bones, and only in relatively very few with a sharpness suitable to a rapid, certain 

 and at the same time, practical determination of the age in practice. The investigation 

 showed further, that in the bones when air-dried, just as in the otoliths, two layers always 

 belong to each year's ring, one white and one dark and transparent by reflected light. 

 The white layer contains more bony cavities and is, correspondingly, relatively poorer than 

 the dark layer in connective tissue with fixed calcareous matter. The yearly growth, 

 which probably ceases completely in winter, as in trees, begins always with the formation 

 of a white layer, then is continued with the dark layer and ends usually with the form- 

 ation of boundary lines more a less sharply marked. 



The bones which display the age-layers most clearly are, in general, the vertebrae, 

 especially the inner aspects on the walls of the cavities between the vertebrae, further the 

 flat bones of the gill covers, especially the sub- and interoperculum, lastly, certain parts of 

 the bony shoulder girdle and pelvic girdle, especially the coracoid and scapula. To these 

 may be added, the so-called urostyle and the neighbouring bones, forming with it the end 

 of the vertebral column, viz. the epural and hypural bones, certain parts of the suspensory 

 apparatus of the jaw, the hyomandibular. 



It is not the same bones of the skeleton in all species, which show the age-layers 

 most distinctly. In the plaice, it is the vertebrae — the rings on the inner side of the 

 cone — and the sub- and interoperculum, which determine the age almost as clearly as 

 the otoliths. This holds also for the turbot. In the sole, it is practically only the verte- 

 brae that can be used. In the gadoids, the flat bones of the pectoral girdle, the coracoid 

 and scapula, are best suited for the determination of the age, especially in the cod which, 

 for the rest, is one of the most difficult species on which to determine the age. For the 

 haddock, the vertebrae especially are very useful, and quite exceptionally, likewise the 

 rounded swollen lower part of the Clavicula of the shoulder girdle, characteristic of this 

 species; a section across this bone is usually sufficient for a certain determination of the 

 age. In the herring again, the vertebrae are very suitable, also, especially for the 0-, I- 

 and Il-groups — the bones of the gill-cover. 



The proof, that these layers in the bones are just as good age-layers as those of the 

 otoliths, can be undoubtedly displayed by us and others from numerous comparisons, e. g. 



