PTERICHTHYS. 93 



consists of an anterior and posterior dorso-lateral plate, the "anterior" and 

 " posterior lateral " of Hugh Miller and Sir P. Egerton, with whose previous 

 observation I am here in accordance. Now, these two plates are traversed in 

 antero-posterior succession by a continuous longitudinal groove, cut obliquely into the 

 bone from below upwards, so that the upper margin overhangs as it were, and this 

 groove is also continuous in front with the lateral line groove on the head, and is 

 consequently to be considered as part of the same system. 



By Professor Jaekel, on the other hand, this groove is looked upon as the 

 remains of a suture separating each bone into two elements, upper and lower, 

 those in the case of the anterior plate being interpreted as " operculum" and 

 " suboperculum," while the posterior one is in like manner dismembered into 

 " supracleithrum" and " cleithrum." The question as to the possibility of homolo- 

 gising the dermal plates of Asterolepidae or any other Ostracoderms with skeletal 

 elements in Teleostomi must be reserved for another chapter of this work ; at 

 present it is sufficient to say that, being unable to accept Prof. Jaekel's homologies 

 for these bones, I see no reason to depart from the system of names, based on 

 that of Pander, which I have up to this time used in this monograph. At present 

 the question is as regards a matter of fact, and here it must also be noted that 

 the grooves in question were looked upon as obsolete sutures by Pander as far 

 back as 1857, and that he was also inclined to consider the set of similar grooves 

 on the cranial bones as belonging to the same category. 1 



Jaekel, however, does not seem to deny that these head grooves belong to the 

 sensory canal system, and if that be the case, the same interpretation must be 

 given to the groove along the side plates, for in perfect specimens of Bothriohpis 

 Canadensis the groove on the lateral occipital of the head can be directly traced 

 into that on the anterior dorso-lateral of the carapace. 



Nor does he deny that on the internal or visceral aspect of these side-plates no 

 line of division is observable in the normal condition, though in compressed 

 specimens there is a tendency for the plate to divide and gape along the line of the 

 groove in question. This is, however, merely the result of pressure along the 

 obvious line of least resistance, and this artificial division never extends to the 

 thickened part of the plate concerned, anterior in the case of the anterior dorso- 

 lateral, posterior in the case of the plate behind. 



Again, Professor Jaekel makes a point out of the occasional occurrence of 

 granulation on what I consider to be the oblique walls of the sensory groove, but 

 which, according to him, are the imbricating surfaces (Ueberschiebungsfidchen) of 

 distinct plates. That this proves anything at all, beyond the fact that we have here 

 before us an open groove, I cannot see. Lastly, he points to a slightly prominent 

 line above, and parallel with the groove in question, and coinciding with the 



1 ' Placoderuieu," p. 50. 



