86 E. T. NEWTON ON FISHES* TAILS. 



hypural plates. In counting the hypural bones in the stickleback, 

 sprat, carp, and flounder, I have regarded as the last haemal arch, 

 the hindermost one "which has a corresponding neural arch fully- 

 developed. The first bone beyond this I have called the first 

 hypural, its neural arch being short and modified. According to 

 this mode of reckoning, the cod has one hypural, the stickle- 

 back and flounder two, the sprat, carp, and salmon seven. 



It is proposed now to glance at the different groups of fishes, 

 and see which of these forms of tails occur in each of them. 



1. Pharyngobranchii. — This group includes but one form, the 

 Amphioxus lanceolatus ; in it the notochord is persistent, and 

 ends in a point in the middle of the tail. It agrees therefore with 

 the earliest stage of the flounder noticed by Prof. A. Agassiz, and 

 is diphycercal. 



2. Marsipobranchii. — The lampreys and hags which compose 

 this group, although much higher in the scale in other respects, 

 agree with the Amphioxus in having a persistent notochord, which 

 terminates in a point in the middle of the tail-fin. These fishes 

 therefore, likewise agree with stage 1 of the flounder, and are 

 diphy cereal. 



3. Dipnoi. — Although the fishes of this group, Lepidosiren, 

 Protopterus, and Ceratodus, are so highly developed in many 

 points of their structure, yet in the form of their tails they are 

 very lowly ; and consequently, in the present instance, are taken early 

 in the series, although mostly placed as the highest group of the 

 fishes, because they show affinity with the Amphibia in having 

 lungs as well as gills. All these fishes have the notochord per- 

 sistent, and ending in the middle of the tail without turning up- 

 wards. They are diphycercal, like groups 1 and 2, and agree with 

 stage 1 of the flounder. 



4. Elasmobranchii. — The sharks, rays, and chima?ra which 

 constitute this group exhibit much diversity as to the extent of 

 ossification of the vertebral column. In some the notochord is 

 persistent, in others the vertebrae are fully ossified ; there is like- 

 wise much diversity in the form of the tail. Concerning this 

 group Prof. Huxley says :* — " The terminal part of the notochord 

 is never enclosed within a continuous bony sheath or urostyle. The 

 extremity of the vertebral column is generally bent up . . . 

 Elasmobranchs with tails of this conformation are truly 



* " Anatomy of Vertebrated Animals, 1871," p. 127. 



