442 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. [DoC. 15, 



very briefly described by Prof. Agassiz, and is figured in pi. 31, figs. 

 13 and 14, of his abeady cited * Monographie.' The specimen is an 

 irregularly broken mass of sandstone, exhibiting numerous impres- 

 sions of four- sided scutes,: of which there are altogether five rows 

 in one direction and eleven in the other. A plaster-cast (PL XIV. 

 fig. 1) shows, even better than the original, that, while one opposed 

 pair, out of the four edges of each scute, fitted against the adjoining 

 edges of the scutes on each side, the other pair of edges alternately 

 overlapped, and were overlapped by, those of the adjacent scutes. 

 There can be no doubt that the overlapped edges were anterior, and, 

 as I shall presently show that these scutes formed part of the 

 ventral armour of the animal to which they belonged, the direction 

 and relations of each row become at once definable. The five rows 

 are longitudinal — the eleven transverse. Kone of these rows are 

 complete. The left-hand longitudinal row contains five scutes, whose 

 outer (left) edges are more or less broken away. The next row 

 contains seven scutes, the posterior of which are somewhat thrust 

 forward, and their left edges somewhat broken. The third row con- 

 tains eight scutes. Each of these longitudinal rows extends to the 

 same level anteriorly ; but the next, or fourth, series begins opposite 

 the fifth scute of the third series, and but a very small portion of its 

 first scute is visible. It contains six scutes, of which the hindermost 

 are somewhat displaced and thrown forwards one upon the other. 

 The fifth series contains only portions of five scutes, which are more 

 or less displaced towards the right side*. 



An additional small fragment of a scute is visible in front of the 

 first and third series. Each scute is on exactly the same level as its 

 right-hand and left-hand neighbour, so that the structure of the 

 whole fragment is extremely regular. 



About a fifth of the outer surface of each scute is covered by the 

 posterior edge of its predecessor ; and the fifth and sixth scutes of 

 the fourth series are sufiiciently displaced to show that the covered 

 surface was smooth and bevelled ofi" obliquely, so as to constitute a 

 sort of articular facet, narrow and parallel-sided antero-posteriorly, 

 but very wide transversely. 



The posterior edge of this facet is cut perpendicularly to the 

 plane of the scute, from whose face it rises Hke a kind of parapet. 

 The face of the scute is ornamented with a peculiar sculpture, con- 

 sisting of distinct deep pits. The casts of these are of course elevated, 

 and lie like drops upon the general surface of the impression — an 

 appearance which doubtless suggested the name of the genus. Near 

 the centre of the face of the scute the pits are nearly circular in 

 outline, but towards the periphery they elongate in the direction 

 of radii from a point rather nearer the anterior than the posterior 

 edge of the whole scute, and assume a pyriform shape, the small 

 end of each being directed inwards. The consequence of this 

 arrangement is a very marked radiation of the ornamentation from 



* I have applied the terms right and left here, not to the true right and left 

 series of scutes, but to those which appear to be right and left when the face of 

 the fossil is turned towards the observer and its anterior end is forward. 



