of the Fisher// Board for Scotland. 



131 



bodies composed of elongated cells : these are sense-organs, which in the 

 cod are freely distributed over the surface of the skin, in addition to the 

 special organs of the lateral line. The derma below the epidermis 

 consists of line fibres having a horizontal direction, with numerous nuclei, 

 and in this occur the scale-pockets containing sections of the scales. Of 

 these, three layers at three different levels are seen in most sections, in 

 consequence of the fact that the scales are imbricated, and overlap one 

 another, so that portions of three different rows of scales are cut by one 

 section vertical to the surface. The relations of the sclerites to the lower 

 homogeneous portion of the scale are shown in the figure, but the cellular 

 investment of the scale is not represented, as it is not sufficiently distinct 

 under a low power. 



The ridges on the scales are seen to be farther apart in some regions, 

 more closely crowded in others, so that zones may be distinguished. In 

 other words, the sclerites formed at some periods of the growth of the 

 scale are narrower than at others, so that their ridges are closer together. 

 There is good reason for believing that the narrower sclerites are formed 

 in winter, when the temperature is low. In the cod I find that the end 

 of a year's growth is usually indicated by one or two markedly narrow 

 rings, while the gradual narrowing of the sclerites as this boundary is 

 approached, though it can be made out, is not at first obvious. In these 

 points my observations agree generally with the descriptions and cenclu- 

 sions of Mr. J. Stuart Thompson, who, however, though he studied 

 several species of Gadidte, did not investigate the cod. In larger and 

 older specimens several annual zones can be distinguished, each termi- 

 nated by a winter zone, between which and the summer growth there is 

 often a distinct contrast. The distinction however, is not so obvious as 

 in the earlier or inner zones, and it would be very difficult, from the scales 

 alone, to form a decided conclusion as to the age of a cod. 



Dr. Heincke has pointed out, in his paper in the Report of the Inter- 

 national Investigations, shortly to be published, that lines of growth are 

 more or less distinctly visible in various bones of fishes, while the lines 

 and zones of the otolith have previously been investigated in the plaice 

 and herring. In the cod I have examined the bones of the pectoral 

 girdle, of the operculum, of the skull, and of the vertebral column. 



The pectoral girdle differs from that of the plaice in several particulars. 

 The cleithrum (the large superficial bone behind the branchial cavity, 

 formerly known as clavicle) is thinner, and only the scapula is directly 

 attached to the posterior and inner aspect of the cleithrum. There is a 

 long and strong post clavicle. The cleithrum is ossified, but the scapula 

 and coracoid consist of calcified cartilage. There is no scapular foramen. 

 There are four distinct and partially ossified fin-supports, instead of one 

 cartilage, as in the plaice. The form of the parts is shown in fig. 20, pi. ix. 

 I could not satisfactorily make out annual zones in any of these structures. 

 On the thin transparent part of the cleithrum there are sometimes visible 

 some lines parallel to the edge, but they are very indistinct. Also in the 

 coracoid there are slightly opaque lines parallel to the edge, but still less 

 distinct, and no definite conclusions as to the age of the fish can be drawn 

 from them. I am unable, therefore, to agree with Heincke, who states 

 that the age of cod can be determined from the coracoid and scapula. 



I have found the bones of the operculum and skull equally unsuitable 

 for the purpose here in view. In the vertebral column of a specimen of 

 some size, parallel lines are distinctly visible in almost every part and on 

 every process, including the walls of the anterior and posterior cavities, 

 the neural spines, and the transverse processes, but I have found it 

 impossible to use them as satisfactory indications of age. On the walls 

 of the conical hollows of the centra numerous concentric lines appear, 



