272 On the Skeleton of the Plesiosaurus Dolichodeirus. 
small holes, for the insertion of the points of a pair of com- 
passes, either of the common kind, of the construction used by 
clock-makers, or that which is known under the name of beam 
compasses. 
The compasses may be used to regulate the opening of the 
sector, or to ascertain by the aid of that instrument, the com- 
parative value of the distances which the rod of the chyometer © 
has to be introduced into its tube. 
In order to convey an idea of the nature of the sector to any 
reader who may be unacquainted with it, I trust it will be suf- 
ficient to point out, that its construction is similar to that of 
the foot-rule used by carpenters. We have only to suppose 
such a rule, covered with brass, and each leg graduated into 
200 equal parts, in order to have an adequate conception of 
the instrument employed by me. 
A more particular explanation.of the principle of the sector, 
may be found in any Encyclopaedia, or Dictionary of Mathe-’ 
matics. 
XLII. On the Skeleton of the Plesiosaurus Dolichodeirus dis- 
covered in the Lias at Lyme, in Dorsetshire, in the Collection: 
of His Grace the Duke of Buckingham. ) 
HE plate (III.) given in our present Number, represents a 
nearly perfect skeleton of the Plesiosaurus Dolichodeirus, 
described by the Rev. W. D. Conybeare, F.R.S. &c. in a me- 
moir given at p. 412 of our 65th volume. The drawing from 
which the original plate in the Geological Transactions was 
engraved was executed with extreme care by Mr. Webster. 
The several parts are described in the memoir. 
** The bones are entirely imbedded in a matrix of lias shale, 
which, though intersected in several places by lines of fracture, 
has evidently, from the mutual adaptation of the parts, formed 
one entire mass. Above twenty of the cervical vertebrae con- 
nected with the head, lie together unbroken. 
‘* We have omitted to state in the memoir, that a second 
unbroken specimen of the entire vertebral column, from the 
head to the tail, was found at the same time and place with 
the one here represented ; and has been presented by Professor 
Buckland to the museum at Oxford.”— Trans. Geol. Soc. Sec. 
Ser. vol. i. 
[See a delineation of the Skeleton conjecturally restored in 
Plate III. vol. lxv.] 
XLIII. Note 
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