310 R. Lydekker—Note on the Ichthyopterygia. 
most, and Mixosaurus the least, specialized genus; and I find that 
while among the Liassic species Ichthyosaurus communis in the 
structure of its limbs makes the nearest approach to Ophthalmosaurus, 
I. tenuirostris and its allies are the forms most nearly allied to Mixo- 
saurus. Now the pectoral limb of the generalized JI. tenwirostris 
having only four digits, while in the more specialized species the 
number is greatly increased, it may be inferred that Ichthyosaurs 
have descended from a tetradactylate ancestor, or at least that only 
four digits have been primarily modified into the Ichthyosaurian 
paddle. A comparison of the pectoral limb of I. tenuirostris with 
that of Chelydra has moreover led me to conclude that the four digits 
there found correspond to the 2nd, 3rd, 4th, and 5th of the typical 
manus ; the 3rd arising in the same way from the intermedium, and 
the 4th and 5th conjointly from the ulnare. The primary grouping 
of the genus which I have adopted is mainly based upon the simpler 
or more complex structure of the pectoral limb; and I may add that in 
those forms where the original four digits have become split up it is 
evident that the presence of two centralia in the carpus is an acquired 
and not an inherited character. ‘This classification is in the main a 
modification of the one proposed by A. Wagner, and subsequently 
extended by Col. Kiprijanoff in the Memoirs of the St. Petersburg 
Academy for 1881. It is briefly summarized in the following table, 
which contains a synopsis of all the named species with which I am 
acquainted. The specific names applied by Hawkins to several of 
the English Liassic species are, however, omitted. In cases where 
the generic position of species is uncertain a note of interrogation is 
placed after the generic name; and when the serial position is pro- 
visional an asterisk is prefixed :—- 
I. Genus Oputuatmosaurus, Seeley (Baptanodon = Sauranodon, 
Marsh).—Humerus articulating distally with three bones. 
1. OPHTHALMOSAURUS ICENICUS, Seeley. Oxford and Kimeridge 
Clays, England. 
2. OPHTHALMOSAURUS NaTANS (Marsh). Up. Jurassic, N. America. 
3. piscus (Marsh). __,, + 
4. i CANTABRIGIENSIS, N.Sp. noble Cambridge, 
Greensand. 
Il. Genus Icutnyosaurus, Konig.—Humerus articulating distally 
with only the radius and ulna, which are short and in close 
apposition. 
A. Latipinnate Group.—Pectoral limb with the third digit (that 
arising from the intermedium) containing two longitudinal 
rows of bones and two centralia; radius very short, with 
entire anterior border. 
a. Campylodont subgroup.— Roots of the teeth enveloped in cement ; 
humerus with prominent trochanteric ridge. 
a. Femur very short, with trochanteric ridge enormously developed. 
5. IcHTHyOsAURUS CAMPYLODON, Carter. Up. Cretaceous, Europe. 
*6. i INDICcUS, nobis. Up. Cretaceous, 8. India. 
eile A Srrompucki, Meyer. Neocomian, N. Germany. 
*8, Bs POLYPTYCHODON, Koken. Neocomian, North 
Germany. 
