156 
DR. MANTELL ON THE FOSSIL REMAINS OF TURTLES. 
carapace of the Emydes, to render it probable that these remains are referable to that 
family of Chelonia which live in fresh water or estuaries*.” 
The specimen discovered by Mr. Bensted, consists of the dorsal shell of a young 
Turtle, with a considerable portion of the plastron, and is adherent to a block of 
chalk by the external surface of the sternal plates. The carapace, which is almost 
complete, is of a depressed ovate form, and is six inches long, and 3| inches wide in 
the middle. It is composed of eight ribs on each side the dorsal ridge, and is sur- 
rounded by a border of marginal plates, which, with the exception of an interval of 
about two inches on the anterior and dextrai aspect ( a a*, Plate XI.), is entire. These 
plates are joined to each other by finely indented sutures, and bear the impress of the 
horny scales, or tortoise-shell with which they were originally covered. The ex- 
panded ribs are united together throughout the proximal half of their length, and 
gradually taper to their marginal extremities, which are protected by the plates of the 
osseous border. Such are the characters presented by the specimen, as shown in 
Plate XI. But Mr. Bensted has with great skill dissected away the chalk, so as to 
admit of the removal of a great portion of the dorsal shell (from a * to b, Plate XI.), 
and thus some of the vertebrae, four plates of the plastron, and a coracoid bone, are 
brought to view, as seen in Plate XII. fig. 1. The inner surface of the carapace is 
also thus displayed (Plate XII. fig. 2.), together with the mode of union and growth 
of the costal processes, and the attachment of their distal extremities to the osseous 
border. The accuracy of the drawings renders any detailed description unnecessary. 
The number and variety of existing Testudinata are so great, that it is scarcely pos- 
sible to determine, from the remains of the cuirass alone, whether a fossil Turtle be- 
longs to an extinct species. It might seem easy to decide whether it were referable 
to the marine or lacustrine Chelonia, but even this question is not readily solved, when, 
as in the present instance, characters usually supposed to be distinctive of the sub- 
genera, are found blended in the same individual. At first sight, the form of the 
dorsal shell, and the interspaces between the distal half of the ribs, might lead to the 
inference that it is the cuirass of a marine species ; but my friend Mr. Bell, who has 
carefully examined the specimen, and compared it with the recent forms in the Hun- 
terian Museum, and in his own matchless collection of Testudinata, refers it to the 
Emydidse ; an opinion in accordance with that entertained by Mr. Owen of the re- 
mains above mentioned, as having also been procured from the chalk-pit at Burham. 
Mr. Bell observes, that “ the manner in which the ribs diminish in breadth to- 
wards the circumference of the carapace, indicates a gradual growth, and a tendency 
towards the complete filling up of the interspaces ; very different from that observable 
in the marine Turtles, in which the broad portion terminates abruptly, and the 
narrow part is almost linear in its proportions.” The annexed sketches will serve to 
exemplify these remarks. Fig. 1. represents the termination of the expanded part of 
the ribs in the marine Turtles ; fig. 2. the corresponding processes in the fossil. 
* Proceedings of the Geological Society. 
