M. E. Storms on tlie Adhesive Disk o/'Echcnels. 71 



A. An expanded and horizontal upper part, transversely 

 enlarged in two places, which gives it the form of an X. 

 The first enlargement carries the spinous ray. 



B. The lower part, forming the shaft of the bone, directed 

 downwards and forwards between the lateral muscles. 



2. The upper interneural spine^ which can be proved to be 

 the haseost (Cope and E-yder), has the shape of two wings. At 

 tlieir junction these carry a process directed backwards, which 

 enters into a cavity at the base of the spine. 



3. The sjmious ray has a transversely much enlarged base, 

 in the middle of which there is a small hole. The base 

 carries on its posterior side two processes directed backwards 

 when the ray is erected. A groove is seen on the spine itself, 

 showing tlie two halves which formed the ray. 



III. Comparison hetween the Elements of the Disk and those 

 of a Spinous Dorsal Fin. 



1. Interneural spine. — There can be no difficulty as to the 

 homology of the interneural spines in the disk and spinous 

 dorsal fin, as admitted by MM. Beck ■^, Niemiec f, &c. 



2. Intercalary hone {Fussjjlatt, Beck; Os trabeculaire, 

 Niemiec). — This bone corresponds by its position on tlie 

 interneural spine (it rests not on the anterior widened surface 

 of the interneural spine, but more backwards and connects each 

 interneural with the following), and especially by its winged 

 shape, to the upper interneural (or baseost) of Scomber. 



3. Pectinated lamcllce [Zahnplatten, Beck; Lames pectinees^ 

 Niemiec). — These bony lamellee have been considered by most 

 authors to be transformed spinous rays. They correspond to 

 spinous rays by their position on the interneural spines (each 

 pair of lamellge rests at their junction on the enlarged upper 

 surface of the interneural spine) , by their position with regard 

 to the intercalary lamellae, and by the strong processes which 

 they carry underneath. To this interpretation M. Niemiec | 

 objects that it is very difficult to explain the transformation 

 of a median bone into two lateral symmetrical bones differing 

 completely from the primitive one. But it ought first to be 

 borne in mind that spinous rays, like the articulated rays, 

 are formed of two halves § (traces of this can still be seen in 

 the spinous rays of Scomber) more or less completely joined. 

 It is easy to conceive that these can remain separated by 

 arrest in development. 



* G. Beck, loc. cit. p. 31. t Nieiniec, loe, cit. p. 116. 



\ Niemiec, loc. cit. p. 117. 



§ Wiederslieim, ' Lelirbucli der vergleicbeuden Auatomie,' p. 168. 



