FOSSIL FISHES. 13 



tion, as we know was the case in Diniohthys. The different plates of the 

 body buckler in Ooooosteus are usually supposed to have been somewhat 

 firmly united. By Agassiz, Pander, Miller, and Owen, they are repre- 

 sented as being all soldered together, but as they were plainly united 

 by splint joints — the margins in some instances greatly overlapping, and 

 the component bones separating so easily that they are usually found dis- 

 connected—we are compelled to believe that they were less firmly joined 

 than the head bones. 



The Supra-Scapulas (Post-Temporals of Parker) of Coocosteus are very 

 much like those of Diniclithys ', being similar in form, and having a cor- 

 responding articulation with the angle of the head. The only noticeable 

 difference in this articulation in the two genera is the thumb-like process 

 thrown out from the epiotic (?) bone, to strengthen it, in Diniclithys. This 

 is wanting in Ooccosteus, but the linear furrows forming the large-figured 

 ornamentation, described elsewhere, is visible on the "Post-Temporals" 

 and Epiotics(?) of both genera.* The "Post-Temporals" of Chelyo- 

 pliorus are still more like those of DinicMlnjs ; scarcely differing in any 

 respect, except in size. 



In the present volume are published figures of the dorsal shield of Cog- 

 costeus, from American and Scotch specimens ; and the dorsal shield of 

 Dinichthys is represented on one of the large plates (Chart V.) which 

 accompany this report. From the figures on this chart, all of which are 

 of the natural size, a comparison may be readily made, and it will be 

 seen at a glance that the difference is considerable. That the bones 

 under consideration are homologous, there can be no reasonable doubt ; 

 but we must turn to another group of Placoderms to find dorsal shields 

 like that of Diniohthys. These we meet with in Asterolepis and Heterus- 

 lius, especially the latter. The dorsal shield of Aaterolepis is the bone 

 described by Hugh Miller as a hyoid plate. Pander, however, places it 

 in its true position, on the back, immediately behind the head. Bj refer- 

 ence to the figures given on Plate 8 of his work, cited above, it will be 

 seen that in all essential points of structure the dorsal shields of Heteros- 



* The bone of fishes, called the Supra-Scapula by Cuvier, and by most zoologists since 

 his time, is asserted by Mr. W. K. Parker to be the Post-Temporal, and since the oblong 

 quadi-angular bone which is articulated by a very perfect and movable joint with the 

 head in Coecosteus is named the Supra-Scapula by Prof. Huxley, Mr. Parker calls that 

 also the Post-Temporal. While not prepared to deny the accuracy of this view of an 

 anatomist so deservedly distinguished as Mr. Parker, I venture to ask for this case a 

 rehearing, and the consideration of the suggestion that this bone, so entirely indepen- 

 dent genetically from the cranium, belongs to a posterior vertebral arch ; and that it is 

 either what it has been called, the Supra-Scapula, or still better, the Supra-Clavicle. 

 With this interpretation, the bone with which it articulates, and which forms the pos- 

 terior lateral angle of the head would be the Post-Temporal. 



