60 



OLD RED SANDSTONE FISHES. 



Siraiicjrajjliical Position.- — In Red Sandstone, immediately above the ' Auclieiiaspis 

 Grits ' of Ledbury. 



Characters and General Remarks.- — See woodcut, fig. 33. The fossil is in two 



pieces, one showing the convex cast (PI. XIII, fig. 2), the other concave surface 

 (PI. XIII, fig. 1), of the cephalic shield. This shield is of an oval form, about an inch in 

 length and a little less in breadth. Anteriorly it is divided by a well-marked junction- 

 line into two portions — an anterior semicircular piece and a posterior larger and somewhat 

 square piece. The anterior portion exhibits two distinct oval orbits, placed close together 

 at its centre. Radiating channels mark the inner surface of this portion of the scute, 

 recalling the similar 'channeling' in Cejjhalaspis (also seen \n Auchenaspis, being the 

 structure which gave rise to the supposition of a fibrous bony skull in Agassiz's descrip- 

 tion of Cejjlialasjns). The line of junction between the two plates describes a double 

 curve ; the two produced angles of the semicircular plate embracing the posterior plate, 

 but not diverging from it, whilst in the median line the anterior piece is produced into 

 the posterior to a small extent : in this way the double curvature of the margins is 

 effected. Posteriorly the posterior j^late becomes contracted, and its margins tend towards 

 describing a dome-like outline when it is abruptly truncated, and the truncated margin 

 thickened and inflected. A thickened ridge passes anteriorly along the median line. No 

 radiating channels mark its inner surface. Plake-like fragments from the inner substance 

 of the scute, soaked in Canada balsam and examined beneath the microscope, show large 

 bone-lacuuce very densely packed, arranged at right angles in the different lamellae of the 

 I)ony material, so as to produce the appearance of cross-hatching. 



The small specimen figured, which is from Dr. Grindrod's great collection, is the 

 only relic of this form Aviiich we at present possess. 



A. In PI. XIII one or two fragments of scales are figured which may possibly be 

 connected with Cephalaspids, or with the bearers of those ichthyodorulites which not 

 unfrequently occur in the Cornstones of Herefordshire and subjacent beds, and have been 



designated Onclnis. 



I would particularly draw attention to the beautiful fragments which I have termed 



Fig. 33. 



Diagram outline of the head-shield of 

 Didymaspis Grindrodi. 



