Lankester — On a new Fossil Fish. 153 



the red, compact Sandstones, immediately resting on the grey 

 ' Passage-beds ' at Ledbury. Dr. Grrindrod has kindly lent it to 

 me for description. 



The fossil is in two pieces, one showing the convex surface, 

 the other the concave impression, of the cephalic shield of a fish, 

 evidently closely allied to CepJialaspis. This shield is a little above 

 an inch in length, and a little less in breadth. Anteriorly, it is 

 transversely divided by a well marked line of junction into two 

 portions; an anterior semi-circular piece, and a posterior, larger, 

 and somewhat square piece. The anterior portion exhibits two 

 distinct oval orbits, placed close together at its centre. Eadiating 

 channels mark the inner surface of this portion of the scute, re- 

 calling the similar ' channelling ' in CepJialaspis. The line of 

 junction between the two plates describes a double curve, the 

 two produced angles of the semi-circular 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 plate 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. 



Flake -like fragments from the inner substance of the scute, 

 soaked in Canada-balsam, and examined beneath the microscope, 

 show large " bone-lacunae" very densely packed, arranged at right 

 angles in the different lamellas of the bony-material, so as t 

 produce the appearance of cross-hatching. (See Plate VIIL, Fig. 7 



From these characters, it is sufficiently clear that this head-shield 

 belongs to a member of the Cephalaspid(^, and of that section of the 

 family in which the calcareous material presents true bone-lacunge. 

 It resembles most closely the Auchenaspis of Sir Philip Egerton, 

 which has two species to represent it ; it differs from this genus in 

 the absence of diverging cornua from the anterior portion of the 

 shield, in the relatively larger size of the orbits, and in the very 

 different form, and far greater relative size, of the posterior or neck- 

 plate. Hence I propose to call this form Didymaspis (S/So/^o? 

 twin, aa-irl'i shield), alluding to the two nearly equivalent portions 

 into which the shield is divided. This species I wish to call D. 

 Grindrodi, after the indefatigable investigator to whose collection it 

 belongs. There appears to be a distinct homology between the 

 anterior portion of the shield in Auchenaspis aud Didymaspis, and 

 the rostrum, orbital pieces, and cornua, of the genus Pteraspis (the 

 most elaborate form of that section of Cephalaspids in which the 

 shield presents a cancellous structure, with no bone-lacun£e), 

 whilst the "neck" plate of Auchenaspis, and larger posterior plate 

 of Didymaspis, coiTcspond to the disc and spine of Pteraspis. In 

 Cephalaspis, on the one hand, and Scaphaspis^, on the other, these 



^ Scaphaspis is the genus allied to Pteraspis, which includes the species Lloydi, 

 trtmeatus, etc., with a simple ovate shield. 



