458 



AMERICAN MUSEUM OF NATURAL HISTORY 



Fins — Cont'd. 



In Cladoselache, a primitive Elftsmobranch, 

 the radial elements have become divided into a 

 basal and a distal segment (Dean, B. 1894.1). 

 In existing Elasmobranchs, the elements are 

 generally trisegmental. While generally triseg- 

 mental in the Teleostomes, they here exhibit a 

 tendency toward reduction, and their original 

 relations to the vertebral arches are obscured. 

 They are, in Teleosts, generally called " inter- 

 spinous bones " (interneural or interhgemal 

 elements) . 



The series of endoskeletal fin-supports, fol- 

 lowing the arrangement of Bridge (p. 533), 

 may be termed radial elements. They may be 

 further subdivided into proximal (basal or in- 

 nermost), mesial, and distal segments. When 

 bisegmental, the terms " basal " and " radial " 

 (Dean, B. 1895.4, p. 32) have often been em- 

 ployed. The terms of Cope (E. D. 1890.1, p. 

 414) ; axonost equivalent to *' proximal", and 

 haseost, supporting the dermal fin-rays, and 

 equivalent to " mesial " or " distal," although 

 adopted by Woodward (A. S. 1889.2, vol. ii, 

 p. 321) seem unnecessary. 



The most valuable summary of this sub- 

 ject {in Ganoids and Teleosts), is I^Bridge, 

 T. W. 1896.1. 



In Elasmobranchs, Mivart termed these 

 elements " radials " (further subdivided 

 into basal or innermost, median, and distal 

 cartilages). Excellent descriptions with 

 figures of riumerous species are given in 

 Mivart, St. G. 1879.1. 



Thacher (p. 292), noting that the fin- 

 supports of Dipnoi are elongated neural 

 spines, believed that this condition, instead 

 of being primitive, was due to the loss of the 

 " primordial median fin-rays " (radials). 

 Thacher, J. K. 1876.1. 



Development of dermal fin-rays 



The following remarks present a summary of 

 Goodrich, E. S. 1904.1. 



In all of the fins of the true Pisces, there are 

 dermal (exoskeletal) rays of mesoblastic origin, 

 the dermotrichia. In Amphio3cus and the Cyclo- 

 stomata, however, true dermal rays are not 

 developed. _ The ray-like structures which have 

 been described in the larva of Amphioxus are 

 merely elongated epidermal cells. The delicate 

 cartilaginous rays supporting the fins of Cy- 

 clostomes are endoskeletal, being prolongations 

 of the neural and haemal arches. 



In the Elasmobranchii and Holocephali, 

 probably also the Acanthodii and Ichthyotomi, 

 there is a single type of dermal ray, the cerato- 

 irichia (Goodrich's term, p. 469), known also 

 as "Hornfaden" or " Elastoidinfaden " by 

 German authors. These fibrous, flexible, horny 

 rays are never ossified nor jointed, but may be 

 branched. The fibrils are secreted by the meso- 

 blastic scleroblasts. Sections of the fin may 

 show several layers of ceratotrichia on either 

 side. The dermal or placoid (shagreen) denti- 

 cles are quite independent of these rays and are 

 more superficial in position, generally conceal- 

 ing them from view. 



In the Teleostomes, occur two types of dermal 

 rays. The first of these, slender, unjointed, 

 horny fin-rays, essentially like and possibly 

 identical with the ceratotrichia, were first noted 

 in the adipose fin of Salmonidse by La Valette 

 St. George (A. J. 1880.2). By Ryder (J. A. 

 1885.6, p. 157) they were named actinotrichia. 

 They are the earliest rays in the embryonic 

 fins, being later functionally replaced by the 

 followmg type, except in the adipose fin and in 

 the growing fin-edges, where they persist. The 

 definitive bony fin-rays, typically ossified, 



jointed, and branched, are termed lepidatri.chia 

 by Goodrich, p. 472. The essential resemblance 

 of these rays to the body-scales was noted by 

 Baudelot (E. 1873.3), and this homology was 

 clearly estabUshed by Hertwig (O. 1876.1). A 

 perfect transition between the scales and the 

 joints of the lepidotrichia in the fossil Amblyp- 

 terus is shown by Goodrich (fig. 55). Formed 

 bilaterally, and later fused, each dermal ray is 

 double. Each ray articulates with the ex- 

 tremity of one endoskeletal fin support (radial 

 element) in the higher Teleostomes, but, in 

 the lower Teleostomes and Dipnoi the dermal 

 rays are more numerous. 



In. the Dipnoi, there are jointed, branched, 

 dermal rays, containing bone-cells, which 

 Goodrich (p. 513) calls camptotrichia. These 

 seem to be in a degenerate condition and may 

 represent the lepidotrichia of the Teleostomes. 

 As in certain Teleosts, scales may extend over 

 the greater part of the fins, and thus secondarily 

 overlie the dermal rays. 



As above indicated, the most valuable 

 paper on this subject is ^Goodrich, E. S. 

 1904.1. 



Scholarly treatises on the development of 

 the horny rays of Elasmobranchs and the 

 bony rays of Teleosts are cited under 

 •Harrison. R. G. 1893.1-1805.2. 



Development or structure of the fin-rays 

 in various fishes. -A-Brohl, E. 1909.1; Han- 

 cock, ,J. 1830.2; •Ryder, J. A. 1879.1, 

 1887.5,1890.1; Szily, A. 1907.1; Vail- 

 lant, L. L. 1896.5; Ziegler, H. E. 1908.3. 



"Very thin, structureless " fin-rays in 

 the adipose fin (of Osmerus) were first noted 

 in a brief remark by Gulliver, G. 1872.2. 



The distinction between fin-rays as exo- 

 skeletal from other bones as endoskeletal 

 breaks down on embryological grounds, 

 since both are of mesoblastic origin, ac- 

 cording to Ryder, J. A. 1885.2. 



delation of temperature to the number of 

 fin-rays in Pleuronectidee (reduction in 

 warmer climates). Cox, P. Add. 1903.1. 



Development of fin spines 



In Elasmobranchs the secondary clustering 

 of the dermal denticles or cusps on the exposed 

 anterior fin margins (Dean, B. 1895.4, p. 28) 

 has produced rigid spines or dermal defenses. 



In Teleostomes the development of bony tissue 

 in the anterior dermal fin rays also produces stout 

 spines. While the soft dermal rays exist in all 

 Teleostomes, spines, followed by soft rays, occur 

 only in more speciaUzed forms, which form the 

 old order of Acanihopterygii. 



Structure and development of the spines 

 in Elasmobranchs. Hannover, A. 1868.1; 

 •Koppen, H. 1901.1; Meyer, G. H. 

 1847.1; •Markert, F. 1896.1; •Ritter, 

 P. 1900.1. 



Structure or development in the following 

 Teleosts. — Siluridce. Bliss, R. 1875.1. 

 — Gasterosteus. Ryder, J. A. 1885.3. — 

 Synodontis.^ Vaillant, L. L. 1895.2, 1898.2. 



Articulation of spines in Teleosts, in- 

 cluding the trigger mechanism for locking 

 the spines and the musculature for their 

 erection. Bhss, R. (Batistes) 1872.2; 

 Dbnitz, F. K. (Teuthis) 1866.1, 1867.2; 

 Hilgendorf, F. M. (Monocentris) 1879.3; 

 Mayer, A. F. 1862.1; Sgirenson, W. E. 

 1884.1, 1897.1; •Thilo, 0. 1879.1-1898.1, 

 .3, 1901.1, Add. 1899.1-1910.1. 



