292 Reports and Proceedings : 
Black Shales are associated with the Bone-bed, which is weli de- 
veloped at the latter place. 
From the foregoing observations, the author concludes that the 
upper half of the White Lias of Dorset appertains to the zone of 
Ammonites planorbis (Lower Lias); the lower half, to the zone of 
the Avicula-contorta-beds, or Upper Trias. The Ammonites-planorbis- 
beds are very similar to the beds of the same zone in Warwickshire, 
Somersetshire, and Gloucestershire,—with this difference, that they 
consist of light-coloured limestones and clays. ‘The Avicula-contorta- 
series differs, however, in having a considerable thickness of cream- 
coloured concretionary limestone in the upper part, instead of the 
laminated clays which this zone contains in its upper part in 
Gloucestershire, Somersetshire, and Glamorganshire. 
On THE Gunus Preraspis. By KE. Ray Lanxesrer, Esq. 
HE author commenced with the history and literature of the 
genus, remarking that little had been hitherto published that 
tended to give very definite notions of the various species, whilst 
the genus Cephalaspis, on the other hand, had received considerable 
attention from Sir Philip Egerton. Professor Huxley bad intended 
to work out the species of Pteraspis, and had accumulated a great 
number of specimens for the purpose; but, after publishing a very 
accurate restoration of the Pteraspis restrata, he had let the matter 
drop, and had now very kindiy lent all his material to the author. 
Mr. Lankester then described at some length the affinities of, and 
differences between, Cephalaspis and Pteraspis, remarking that in 
both the shield was an ossification of merely dermal tissue, and 
showed very remarkable characters; in Pteraspis presenting a 
structure which was without parallel in the animal kingdom, and 
which had been very admirably described by Professor Huxley. He 
then proceeded to show that this peculiar form of ossification in 
Pteraspis is accompanied by a peculiar form of suture; the plates, 
which, connected together, form the head-plate, being not joined by 
interlocking sutures, as in Coccosteus and others, but anchylosed. He 
proposed to divide the present genus Pleraspis into three—Pteras- 
pis, Cyathaspis, and Scaphaspis,—distinguished by the following 
characters, and containing the species here enumerated :— 
Preraspis, — Shield composed of seven anchylosed, but distinct, pieces; 
namely, a central disc, a prolonged rostrum, two orbital 
plates, two lateral ‘cornua,’ and a posterior spine. 
Species—Pt. rostrata, Ag.; Pt. Crouchii, Salter MS. 
Cyatuaspis.—Shield composed of fow distinct pieces or plates; namely, a 
central disc, a short rostrum, and two lateral cornua; spine 
reduced to a mere point. 
Species—Cy. Banks, Huxl. and Salt.; Cy. Symondsit, sp. n. 
ScaPHaspis.—No distinct pieces recognizable in the large oval head-plate 
spine represented by the acute termination of the disc. 
Species—Se, Lewis, Ag.; Sc. Lloyd, Ag.; Sc. truncata, Huxl. and Salt.; 
Se, Ludensis, Salter. 
The characters of the Péeraspis Crouchit, an undescribed species 
