N. ROSEN, STUDIES ON THE PLECTOGNATHS. 17 



nating dark and light lines resemble to a certain extent 

 those often found in fish-scales representing distinct zones 

 of growth. That they have quite another origin is evident 

 from what has been stated. Williamson considered the lines 

 of all the triangles to be connected with each other, thus 

 forming angular concentric lines. As to this question I refer 

 to the deseription given above. 



As mentioned above the plates are juxtaposed. They 

 are connected with each other by bundles of fibres, which 

 continue into those running horizontally towards the centre 

 of the plate (Fig. D). The homogeneous portion of the plate 

 is slightly denticulated along the sides of the hexagon, and 

 separated from that of the surrounding plates. 



Unfortunately I have not had any opportunity of study- 

 ing the development of the plates, but I think it is a hypo- 

 thesis, which cannot be far from the truth, to presume that 

 the development takes place in the same minner as stated 

 above for Mola. That the fibrillated portion of the plate is 

 very intricate in Lactophrys is caused by the complication of 

 the interiör stratum of the corium. In Mola the bundles of 

 fibres have a winding loop, and are not collected in lamellae. 

 A consequence thereof is that, when they become densified 

 and calcified to form a portion of the plate, tbe structure 

 of this will be quite another than that found in the corre- 

 sponding portion of a plate of an Ostraciontid, which owes its 

 origin to a stratum of corium, the bundles of fibres of which 

 are arranged in lamellae. In Mola the exteriör stratum of 

 the corium is comparatively well developed and richly vas- 

 cularised, while the homogeneous portion of the plate pre- 

 sents only a few canals. In Lactophrys the above mentioned 

 layer of the corium is reduced, the homogeneous portion of 

 the plate, on the contrary, being well provided with canals. 



Balistidae. 



The integument of the fishes belonging to this family is 

 very well known especially from O. Hertwig's detailed in- 

 quiry. Agassiz, already, has described two distinct layers 

 of the scales, the superior, according to him, being of the 

 nature of enamel. The incorrectness of this view was pointed 



Arkiv för zoologi. Band 8. N:o 10. 2 



