ON THE STRUCTURE OF THE LABYRINTHODONTS. 245 



age. It is therefore highly probable that the branchial respiration of Arche- 

 ffosaurits was not persistent, biit was restricted to the larval state. 



It is somewhat remarkable that while Yon Mej-er interprets these remains 

 as traces of a branchial apparatus, he nevertheless refuses to recognize the 

 zoological significance of such a structure. His comment is, that tlie hyoid 

 itself is a relic of branchial apparatus, yet its presence in the higher verte- 

 brates is not allowed to interfere with their systematic arrangement *. The 

 serial homology of the hyoid and branchial arches, upon which Yon Meyer 

 perhaps relies, would prove too much for his purpose. The study of deve- 

 lopment shows that " the branchial arches have the same morphological value 

 as the hyoid, and the latter as the mandibular arc;"t further, that the tra- 

 beculae cranii (" Schadelbalken " of Rathke) are serially homologous with the 

 visceral arches. If the argument rests, not upon homology, but upon function, 

 it is clear that the common association of branchiostegal rays with the hyoid 

 arch in branchiate vertebrates would not justify us in describing a part whose 

 function in the higher classes is so various as a remnant of branchial appa- 

 ratus. It would be as reasonable to speak of the humerus as a relic of a 

 swimming-organ. 



Until an example is cited of osseous branchial arches in an abranchiate 

 vertebrate, we may regard the presence of such a structure in the young 

 Arcliegosaurus as a remarkable Amphibian character. 



Dermal Armour. — In nearly all the known species of Carboniferous Laby- 

 rinthodonts a ventral armour has been found. The armour consists of very 

 numerous, elongated, osseous scutes, and is generally, perhaps always, confined 

 to the inferior surface of the body between the fore and hind limbs. The 

 scutes are usually dis])osed in oblique rows, which meet at an angle along 

 the middle line and make a chevron pattern. Such an arrangement occiurs 

 for example in P7io//f?o^ffs/e?' J, Urocordi/lus§, and Ichthyerpeton\\. In ^?-- 

 chegosaurus the pattern is reversed in the hinder part of the trunk, so that 

 the rows of scutes in the front part are approximately at right angles to those 

 placed further back on the same side. 



Lepidotosaurus, if a true Labyrinthodont, presents striking deviations from 

 the rest in the character of its dermal armour. But there arc many diiBcul- 

 ties in the way of obtaining an adequate knowledge of this remarkable form. 

 The state of the single specimen hitherto discovered does not permit more 

 than a superficial examination. Messrs. Hancock and Howse % have done all 

 that care and skill can do towards elucidating its structure ; and we cannot 

 but accept, provisionally, their decision that it must be j^laced among the 

 Labyrinthodonts. Nevertheless the difficulties are considerable, especially 

 with respect to the scales or scutes. The oblique and uniform direction of the 

 very numerous and prolonged rows of scales is an argument against Prof. 

 .Huxley's view that they represent a ventral armour shifted (after death and 

 some amount of decay) to one side. Upon that supposition we should expect 

 to find the rows of scales either transverse (an arrangement not yet discovered 

 in any Labyrinthodont) or converging from opposite sides to a straight line 



* " Genau genommen Hesse sicli selbst das Zungenbcin als ITeberrest einer friiheren 

 Athmungsvorricbtung betrachten, unci doch wirkt dessen Gegenwart iiicht storend bei der 

 Classification der hoheren Thiere."— Eeptilien aus dcr Steinkoblenformation, p. 36. 



t Huxley, Croonian Lecture, ' Proc. Eoy. Soc.| vol. ix. p. 433. 



t Huxley, 'Quart. Journ, Geol. Soc' vol. xviii. 



§ From undescribed specimens in the British Museum from Kilkenny. 



II Huxley, ' On a Collection of Fossil Vertebrata ' &c., p. 18. 



^ Nat. Hist. Trans. Northumberland and Durham, vol. iv. p. 219; and Quart. Journ. 

 Gcol. Soc. vol. xxvi. p. 55G (1870). 



