1910.] CAUDAL FIX Ol' Tin: TliLKOSTOMI. 593 



5. Eparal. — This term may l)e reoarded as applying to the dorsal 



liomologue of the ventral hypnral, i. e. any e[)axial element, 

 having direct continuity with the chordal axis, and ])earing 

 one or more fin-rays at its distal end, will be called an epural 

 or epural bone. It is only after the most careful consideration 

 that I have decided to change the definition of this term. It 

 was established by Huxley in 1859, and appears to have been 

 iised by him to indicate those isolated ray-bearing hones 

 situated immediately dorsal to the urostyle in Gasterosteihs. 

 The change 'suggested is based solely on my conclusions 

 respecting the real morphological value of the several fin-ray 

 supports, and, reluctant as I am to make any changes in 

 established nomenclature, I believe the reader will agree that 

 the subject will be much simplified by the change. 



6. Radial. — The term radial is nsed in its nsual sense as synony- 



mous with " interspinons bone " and " somactid " of some 

 writers; it implies a more or less dagger-like bone, primarily 

 having no direct continuity with the vertebral colunni, and 

 suppoi'ting distally one or more fin-rays. 



7. Litst vertebral seyinent.— This term will refer to the hist centrum 



pins urostyle, if such exists. The last centrum is never per- 

 fect, and is often no more than a cone, the apex of which is 

 directed posteriorly. 



3. Morphology of Caudal Fins. 



This section will be descriptive of a selection of caudal fins, and 

 the order in whi^.h tlie}^ are dealt with corresponds essentially 

 with the classification given by Sedgwick in his ' Text-Rook of 

 Zoology.' Owing to lack of time and material, the series is not 

 as complete as one wonld wish ; foi- example, only the Silin'id;e 

 among the Ostariophysi receive attention, and the .Symbranchii 

 are completely omitted. Except for these sub-oi'dei's, however, 

 the types are fairly representative of the others, at least of the 

 Teleostei. Almo.st every example taken presents interesting 

 features, but owing to the impossibility of including, in the 

 present papei', figures of all the species ti-eated, only the barest 

 description of many forms can be attempted at present. 



AciPEXSEU STUiiio. (Ciiondrostei, Acipenseridte.) (Plate XLVII, 

 fig. 2.) 



The Sturgeon presents a definitely heterocercal tail ; the end of 

 the caudal axis from the root of the tail-fin is dii'ected upwards, 

 and the ventral lobe is enlarged, producing a very unsymmetric;d 

 fin. No centi'a. are formed from the notochordal sheath. The 

 ventral lobe of the fin is supported by a very regular series of 

 hypurals ; the name hypural has been applied here, since tlie.se 

 strictures strictly comply with the definition. They are of 

 particular interest, because, in the anterior part of the fin, where 

 they are most strongly de\e!oped. they exhibit a definite segmen- 

 tation : the largc-t show four hucces.-ive seguieutb (/^rt../>.. //(.. and »/., 



