on one another. The first forty-five bear a double costal tubercle, and the remaining centra have a single one. The fifth, sixth, and seventh centra are each .8 inch long ; the fifteenth is rather less than .9 inch. The vertical and transverse horizontal diameter cannot be accurately taken. The length of the hindermost centra is rather under .8 inch, while their transverse horizontal diameter exceeds that of any of the preceding ones. The ribs do not offer any thing peculiar ; towards their vertebral end they are compressed and channelled ; in the flank they have a cylindrical form ; and their ventral ends are flattened.
A pair of flat bones going below the early thoracic ribs are, I infer, from their position, and from their close association with the scapulas, the coracoids. Their form is unusual. They are much more elongated in the direction of the trunk's axis than are those of the Liassic Ichthyosauri with which I have had an opportunity of comparing them, their axial diameter being to their transverse one as 5.3 : 2.8 inches. With this greater length their anterior border reaches much further forwards in advance of the glenoid cavity than in the Liassic forms ; and it, as also the median border, is straight, so that the latter touches its fellow throughout its whole length when the two coracoids remain naturally united, as they do in this instance. The articular end of the scapula is unusually broad.
The paddles, particularly the hind ones, are extremely reduced. Their precise form and composition cannot be learned ; for most of the lesser bones are missing. The humerus is 2.7 inches long, its distal end is 2.1 inches broad, and the diameter of its middle is 1.6 inch. The femur is only 2 inches long ; its proximal end is 1.4 inch broad, its distal end is 1.1 inch, and the diameter of the middle is .9 inch.
This, as I believe, new species, which I propose to call I. enthekiodon, resembles in the slenderness of its snout the two Liassic species I. longirostris and I. tenuirostris, but it is readily distinguishable from these by the following characters. Its snout, relatively to the length of the cranium, is not so long as that of I. longirostris ; its smaller tooth-root is smooth, and not distinctly fluted as in I. tenuirostris; the shape of its coracoid is quite different from that of the coracoid of tenuirostris ; and its spinal column is much stouter than that of either of these species. It resembles both I. tenuirostris and I. longirostris in the preponderence of its fore paddles over its hind ones ; but its fore and hind paddles, so far as I can judge of their size by that of their proximal bones, are very much smaller ; indeed the paddles, relatively to the whole skeleton, are smaller than in any other species known to me.
EXPLANATION OF PLATE XVII.
Skeleton of Ichthyosaurus enthekiodon from Kimmeridge Bay, one twelfth natural size.
