FOSSIL FISHES. 13 



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

 body buckler in Coccosteus are usually supposed to have been somewhat 

 lirmly 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 Coccosteus are very 

 much like those of DlnicJithi/s j 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 thundj-like process 

 thrown out from the epiotic(^) bone, to strengthen it, in Dlnichthys. This 

 is wanting in Coccosteus, but the linear furrows fornn'ng the large-figured 

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

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

 Ijhorus are still more like those of Dmlchthys j scarcely differing in any 

 respect, except in size. 



In the present volume are ])ublished figures of the dorsal shield of Goc- 

 eosteus, from Amej'ican 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 

 uiuler 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 Dinichthys. These we meet with in Asterolejtis and Heteros- 

 iius, especially the latter. The dorsal shield of Asterolejpis 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. Py 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 Ileteros- 



* The boue of fishes, called the Supra-Sca'pula 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 

 quadrangular bone which is articulated by a very perfect and movable joint with the 

 head in Coccosteus 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. 



