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Proceedings of the Royal Society 
bones, and the external sculpture as well of the bones as of the 
scales. For the larger of these two species, there can be no doubt 
of the propriety of retaining the specific name Hibberti , as it is 
clear that its enormous laniary teeth, as occurring in the limestone 
of Burdiehouse, and at first considered by Hibbert to be reptilian, 
originally suggested the name Megalichthys Hibberti to Agassiz, 
which he afterwards altered to Holopty chins Hibberti, after elimi- 
nating the Saurodipterine remains previously confounded with them, 
and to which latter he then limited the term Megalichthys, rather 
unfortunately, as “ Megalichthys ” is the smaller fish ! M‘Coy’s 
R. gracilis is certainly a synonym of R. Hibberti, the apparent 
greater slenderness of the dentary bone being due to the infra- 
dentary plates (not known to M‘Coy) being, as is often the case, 
awanting in the specimen ; and as regards the greater slenderness of 
the anterior laniary, a large series of teeth from Gilmerton shows 
every possible amount of gradation in that respect. For the smaller 
species, whose remains have hitherto been confounded with R. 
Hibberti, the author proposes the name of R. ornatns. 
1. Rhizodus Hibberti , Ag. sp. 
This species must have attained a gigantic size, a detached den- 
tary bone in the Edinburgh Museum measuring no less than. 25 
inches in length. Externally the cranial bones are ornamented by 
a rather fine tuberculation, the tubercles more or less confluent with 
tortuous ridges. The mandible displays the same structure as in 
Rhizodupsis , the dentary element being narrow, pointed behind, 
thick in front, where it carries the anterior or symphyseal laniary 
tooth, the three other laniaries behind being borne upon separate 
internal dentary pieces. Below the dentary, and forming the lower 
margin of the jaw, are three infra-dent ary plates, while posteriorly 
the articular region is covered by a large plate representing the 
angular element. Other determinable bones described in this paper 
are the maxilla, which, as in Rliizodojpsis, only bears small teeth, 
the principal jugular, the opercidum , the clavicle, the infra-clavi- 
cular ; there are others also whose place in the skeleton is not easily 
determinable. The clavicle and infra-clavicular are not tuberculated 
like the cranial bones, but ornamented with delicate reticulating 
ridges and pits ; the posterior superior angle of the infra-clavicular 
