SCIENTIFIC SUMMARY. 
97 
at its base a conglomerate of pebbles from the older rock, has a strike nearly 
at right angles to that of the latter, upon both flanks of which it rests. 
Mr. Hicks has indicated certain points of resemblance between these pre- 
Cambrian rocks and the Laurentian of Canada, the Malvern Rocks, and 
others also of pre-Cambrian age in Scotland and elsewhere ; but he thinks 
it the safest course to abstain for the present from attempting any definite 
correlation of them, and therefore proposes 'to distinguish the two series 
by names referring to the localities in which they occur, calling the older 
rocks Dimetian, from the Latin name of the district of St. David’s, and the 
newer ones Pebidian, after the district of Pebidiog, in which they are very 
characteristically shown. The exposure of the Dimetian series leads him 
to ascribe to those rocks a thickness of at least 15,000 feet ; the Pebidians 
are apparently of considerably less thickness, but they are in most parts 
more or less concealed by the Cambrian deposits which overlie them un- 
conformably. A great mass of the Pebidian^rocks is exposed to the north- 
west of St. David’s, forming a band running nearly parallel to Ramsey 
Sound ; and at the south-western extremity of Ramsey Island they form a 
bold hill about 400 feet high, on the east side of which a fault, with a down- 
throw of at least 14,000 feet, has brought the Arenig beds into immediate 
contact with the pre-Cambrian rocks. (“ Proc. Geol. Soc.,” Nov. 22,1876.) 
American Pterodactyles. — Professor Marsh has recognised the fact that all 
the known Ptei'osauria, or flying lizards of North America, which have 
hitherto occurred only in the Upper Cretaceous deposits of Kansas, and are 
remarkable for their large size, some of them having an expanse of wing 
of not less than 25 feet, are distinguished from their Old World relatives 
by an entire absence of teeth in both jaws. This is the more interesting, as 
it is in these very deposits that Professor Marsh has discovered the remains 
of birds which violate all the zoological proprieties by possessing true teeth 
imbedded in the jaws. Of these Pterodactyles, for which Professor Marsh 
has established the new order Pteranodontia, at least six distinct species, 
have been recognized, five belonging to the genus Pteranodon , which 
amongst other characters, presents the peculiarity of having the scapula 
firmly co-ossified with the coracoid, and having an oblique articular face at 
its distal end*; and one to his new genus Nyctosaurus , in which the above 
characters are wanting. The type of the new genus was previously de- 
scribed by Professor Marsh under the name of Pteranodon gracilis. It 
seems to have been a middle-sized animal, measuring from eight to ten feet 
across the expanded wings. (Silliman’s u American Journal,” Dec. 1876.) 
American Tertiary Mammals. — -The continuation of Professor Marsh’s 
researches upon the remains of Mammalia from the Eocene deposits of the 
Rocky-Mountain region has revealed a new genus of equine mammals, 
allied to Orohippus , but of an earlier and less specialized type, apparently in 
the direct ancestral line, for which he proposes the name of Eoldppus. It is 
distinguished by having the last premolar above and below similar to the 
next premolar in front of it, and not like the first true molar as in Orohippus. 
The dental formula is the same in both. As in Orohippus the fore-foot has 
four, and the hind-foot three digits ; but there is in addition a rudiment of 
the outer, or fifth metatarsal. Two species are noticed, namely Eohippus 
validus from New Mexico, and E. pernix from Wyoming. 
NEW SERIES, YOL. I. — NO. I. K 
