542 PEOP. T. "W. BRIDGE OK THE MESIAL 



(PI. XXI. fig. 4, sp.r.)axe supported by an equal number of simple, 

 laterally-compressed, unsegmented, and widely separated radial 

 elements (r.e.). The latter are somewhat slender, and the slightly 

 thickened upper or distal extremity of each is tipped with carti- 

 lage and forms a globose condyle, which fits into a suitable socket 

 in the expanded base of its finlet-spine. The radial elements 

 supporting the multiarticulate posterior series of fin-rays (fig. 5) 

 are similar to those supporting the finlets, except for their greater 

 leugth and more cylindrical shape. They are also more concen- 

 trated, and, instead of articulating with their rays by a ball-and- 

 socket joint, the cleft base of each ray simply embraces the 

 cartilage-tipped distal end of its supporting radial element. 

 Most of the anterior radial elements are very obliquely dis- 

 posed, their inner extremities being directed, forwards and only 

 to a slight extent downwards, so that practically the arrange- 

 ment of these elements is nearly horizontal. More posteriorly, 

 where the finlets become replaced by a continuous dorsal fin, the 

 radial elements gradually become less horizontal, and, while still 

 remaining obliquely disposed, approximate more to the vertical 

 and interdigitate with the neural spines of the subjacent ver- 

 tebrae. All the radial elements are embedded in the median 

 vertical fibrous septum separating the dorso-lateral musculature 

 of opposite sides of the body, and by it are connected with one 

 another. 



Thin, somewhat triangular, cartilaginous laminse (fig. 5, x) are 

 attached to the posterior margins of more or fewer of the radial 

 elements, near their outer or distal extremities. These laminse 

 first make their appearance on the ninth, and gradually increase 

 in size to the fifteenth. Prom the fifteenth to the twenty-first 

 they diminish in size and finally disappear, the last one or two 

 elements exhibiting no trace of them. In the twelfth to the 

 fourteenth elements, inclusive, the laminse become more or less 

 completely ossified. Whether osseous or cartilaginous, the 

 laminse project backwards from the various radial elements with 

 which they are in ligamentous connexion into the fibrous septum 

 separating the dorso-lateral musculatures. These structures 

 can scarcely belong to the category of radial elements, and are 

 probably mere chondrifications of the intermuscular septum, 

 developed for the purpose of strengthening the points of origin 

 of the powerful erector muscles of the spines and fin-rays. At 

 any rate the erectores of each spine or fin-ray take origin not 



