Interlocking of the Neural Arches in Ichthyosauria. 44:1 



tail-fin became shorter still. It is not without interest to 

 find the exlremity of the tail presenting so little variation in 

 essentials of structure, as the order of animals is traced 

 through the secondary strata. 



LXIX. — On the Interlocking of the Neural Arches in Ichthyo- 

 sauria. By H. G. Seeley, F.R.S., F.G.S., King's 

 College, London. 



The neural arches in Ichthyosaurs are never closely united 

 with the centrums. When the centrums are isolated the 

 arches are commonly lost. The characteristics of the arch 

 are imperfectly known. They are illustrated by Cuvier in 

 plate cclvi. Oss. Foss., but the figures are indefinite as to the 

 interlocking of the arches, though the text indicates that 

 Cuvier had seen and knew the structural relations of the 

 bones. The excellent preservation of the vertebral column, 

 with the vertebrae in natural sequence in skeletons imbedded 

 in Lias slabs in the museums of this country and the Con- 

 tinent, is unfavourable to demonstration of the mutual 

 relations of the neural arches. Exposed in side view they 

 have a general resemblance to those of porpoises, for there is 

 a manifest contact between them above the region of the 

 neural canal by surfaces which enable the arches to support 

 each other. But in Ichthjosaurus this zygapophysial surface 

 does not project laterally, so that the lateral aspect of a neural 

 arch is smooth and slightly concave from above downward, 

 or only slightly tumid in the zygapophysial area. 1 have 

 never seen any trace of laterally developed zygapophyses 

 except in the cervical region. 



It would appear that the cervical vertebroe is the part of 

 the column most easily observed. Sir Richard Owen, in his 

 ' Keport on British Fossil Reptiles,' 1839, p. 100, speaking 

 generally, states that " the ncurapophyses are interlocked 

 together by means of coadapted oblique processes." Tiiis is 

 true for the neck, but not for the later vortebne. In the 

 account of Ichthyosaurus platyoJon it is remarked in the same 

 memoir : " the articular processes for mutual interlocking 

 are well developed, especially at the anterior part of the spine." 

 Forty-two years later lateral zygapophysial facets in Ichthyo- 

 saurus were figured by Owen in the Palajontographical 



