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Show 1888.] ANATOMY OF THE MESOSUCHIA. 425 dilian basilar piece + the two pleurocentra -f- the pars odontoidea. As bearing on this it is not without significance that the lateral surface of the atlas in Ichthyosaurus is impressed by a diapophysial and a parapophysial pit, for the double costal articulation, as occurs in the axis and the other vertebra behind it. There remains for discussion the inverted V-like piece that caps and superiorly closes the neural arch. As already said, this is missing in Mr. Leeds's specimens, but of its former presence no doubt may be entertained. What is its morphological import ? Cuvier's view that it represents the proc. spinosus of other vertebrae was the doctrine generally accepted until about 10 years ago, when P. Albrecht advanced reasons for regarding it as a vestige of a vertebra ancestrally present between the atlas and the skull, but since suppressed. To this he attached the name proatlas. Albrecht's principal ground for this conception of the nature of the " piece superieure " appears to be the emergence of the first spinal nerve in front of the neural arch of the atlas, for which reason it is by some named suboccipital nerve, whereas all the other spinal nerves escape from the neural canal behind or through the neurapophysis of the vertebra to which they serially correspond. An approximately vertical plane laid through the point of emergence of a spinal nerve will divide the neurapophysis into an anterior part bearing the praezygapophysis, and a posterior portion supporting the postzygapophysis and the spinous process. The neurapophysis appears to have two roots, of which the posterior may be ligamentous, and the nerve passing out between these leaves the neural canal not, Albrecht says, interver-tebrally as commonly taught, but vertebrally by piercing through the neurapophysis, which point of exit is morphologically interver-tebrally situated. Now the vertebral complex called the atlas lies behind the first spinal nerve, and since the serial correspondence of the spinal nerves and vertebrae expressed in numerical order is not as 2 : 2 or 3 : 3, but as 2 : (2 - 1), or 3 : ( 3 - 1) ; or, to express the same circumstance another way, since the second and third spinal nerves correspond respectively to the vertebrae next in front of them, it follows that the first spinal or suboccipital nerve does not correspond to the atlas, but to a vertebra serially in advance of this. A vestige of such an anterior vertebra Albrecht discovers in Cuvier's piece superieure. This he regards as representing the neural arch of the ancestrally present, now suppressed, vertebra once interposed between the atlas and the occiput (31 a). This superior element was subsequently discovered by Albrecht in Hatteria (32). Dr. G. Baur has found it present in Chameleo, sp. (33). Prof. O. C. Marsh has observed its presence in Morosaurus and Brontosaurus (34). L. Dollo also has noticed it in Iguanodon (35). Its presence seems always associated with incomplete coalescence and synostosis of the two sides of the neural arch, and with the absence of a normal spinous process; and this is not without significance, for it hints that after all Cuvier's view respecting it may express the truth. The development of the " piece superieure " in two halves and its discontinuity from the atlantal neurapophyses are not irreconcilable with such |