48 



HAV. 



[Vol. II. 



■ Xiphac- 



processes the basal pieces are excavated to receive the bases of 

 the neural arch, as shown in the figure. The two basal pieces 

 of each vertebra are distinct. Together they seem to form a 

 saddle in which rides the neural arch. 



I find this same structure of the neural arches in some of the 

 vertebrae belonging to the specimens in the United States 

 National Museum ; but in one section of connected vertebrae 

 an arch like those above described is succeeded 

 in the next vertebra behind by an arch in which 

 every trace of a suture between the arch and the 

 apparent basal piece is lost. This vertebra is 

 shown in Fig. 13. The form of the base of the 

 arch is not greatly different from that of the arch f,^, ,3 

 with accessory piece in Fig. 1 1, and we may even *^'^"^- Transitional 



■^ neural arch, x j. 



convince ourselves that we can trace a part of the 

 boundary line between the two portions. There is evidently 

 at this point of the vertebral column a sudden change from 

 neural arches furnished with basal accessory pieces to arches 

 without these, or consolidated with them. Further backward 

 the form of the arches becomes modified somewhat, so that 

 they resemble the one shown in Fig. 14. A section 14 inches 

 long and containing 7 vertebrae having arches of 

 this kind is before me. This condition shows us 

 that the neural arches which are provided with 

 basal pieces are confined to the anterior or middle 

 portion of the tail region, while the hinder portion 

 contains no such vertebral structures. We are 

 reminded that in Amia the middle portion of the 

 caudal vertebral column is composed of two rings 

 for each muscular segment, while the anterior and 

 posterior portions have vertebral centra of the ordi- 

 nary kind. It seems as if the tail portion of the 

 vertebral column of the Amioid fishes and of the 

 Fig. 14. -xi- Isospondyli retained primitive conditions longer 

 phactinus. Neu- ^^i^in thc abdomlual portion. 



ral arch without 



accessory piece. It is difficult to determine what explanation is to 

 ^ *■ be given of the presence of these basal pieces. 



The so-called zygapophyses of fishes are regarded as being 

 outgrowths from the neural arches, exogenous and not autoge- 



