1869.] EGEKTGK^ GYRODUS. 381 



The specimen under consideration was found by the Rev. Mr. 

 Joass, of Golspie (to whom we are indebted for the discovery of 

 the rich deposit of Devonian Ichthyolites at Edderton in Rosshire), 

 on the beach between Culgower and Portgower, on the east coast 

 of Sutherlandshire. It is imbedded in a dark-coloured shale per- 

 forated by recent Pholades, and incrusted in places by Balani. 

 The head and tail are deficient ; but the trunk is tolerably well pre- 

 served, with the exception of the fins. It measures eleven inches 

 in length bj seven or more in depth : as the ventral scales are 

 absent, the latter measurement is merely an approximation. Fortu- 

 nately two of the tritoral teeth are preseired. Although not in situ, 

 there can be no reason to doubt that they belonged to the specimen. 

 They present all the typical characters of the teeth of the genus 

 Gyrodics, viz. a small central papilla surrounded by two crenulated 

 rings, the inner one rather higher than the outer one, having the 

 appearance, viewed vertically, of a small rosette (fig. 1). Judging from 

 their circular form and relative size, it is probable that they be- 

 longed to one of the secondary rows of the vomerine series. The 

 column, as is the case in all the members of this family, is noto- 

 chordal. The neurapophyses, especially in the nuchal region, are 

 strong, and extend nearly to the dorsal ridge. They are spathu- 

 late at the lower extremities, where they partly embrace the chorda. 

 The impressions of a few of the interspinous ossicles supporting the 

 rays of the dorsal fin show that this organ occupied nearly the 

 centre of the back. No evidence remains as to the position or 

 characters of the other fins. The whole of the trunk is invested 

 with a compact scaly armour, remarkable for the solidity of the 

 component scales, and the strength of the joints by which they are 

 united. In most of the Ganoid fishes the scales are united by a pin- 

 and-socket lock, each scale having a process at the upper anterior 

 angle, which is received in a depression on the lower angle of the 

 scale immediately above it in the series ; but in this case each scale 

 has a superior and inferior process, which are spliced to the cor- 

 responding processes in the contiguous scales above and below ; in 

 addition to which, additional strength is imparted by a broad over- 

 lap at each joint, the upper scale covering the bevelled margin of 

 the succeeding scale below (fig. 2). When viewed on the inner 

 surface, these processes form the continuous rib or fillet which has 

 been alluded to before. Gyrodus is the only genus of the secondary 

 formations in which this structure prevails behind the dorsal and 

 anal fins ; this peculiarity has therefore been taken as a good fea- 

 ture for rectifying the generic nomenclature of the several species. 

 The genus Microdon of Agassiz (as I pointed out in 1849) com- 

 prised some species in which the peculiar scale -structure was par- 

 tial, and others in which it was general ; the latter species, namely 

 Microdon heccac/onus and M. analis, are now correctly arranged by 

 Heckel under the genus Gyrodus; on the other hand, Gyrodus micro- 

 pterus of Agassiz diff'ers from that genus in having no scale-ribs on the 

 posterior half of the body, as also in having very large dorsal and anal 

 fins and a rounded tail. These characters combined justify Wagner 



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