Prof. Swinnerton — The Facial Suture of Trilobites. 103 



Some portions of the Asterolepid cancellous tissue belong to plates 

 of considerable thickness. They may perhaps represent a Hothriolepis 

 with a large doi-sal crest, such as has been described by Traquuir in 

 a species from the Upper Old lied Sandstone of Elgin. 1 



Holopttchius. — One Holoptychian scale, shown in impression, is 

 ornamented with very coarse rounded ridges, which are closely 

 arranged and are not subdivided into tubercles. Another specimen, 

 in which only a fragment of the original scale remains, shows by 

 a clear impression that the inner face is destitute of a median 

 tubercle. 



Polyplocodus. — There are several isolated small Rhizodont teeth, 

 which agree with those from the Upper Devonian of Russia and 

 Scotland commonly named Polyplocodus? They are not compressed 

 to sharp edges, but rounded in section, and their outer face is 

 marked by fine vertical grooves or striations. A microscope-section 

 of two specimens shows the typical Rhizodont structure, and one 

 broken tooth proves that the pulp-cavity extends upwards nearly to 

 the apex. Part of the impression of one of these teeth is seen in 

 a piece of Bittadon felsite. 



Coccostean. — Some portions of tuberculated plates are probably 

 Coccostean, but not sufficient for exact determination. 



Conclusion. — The fish-fauna represented by the fossils from the 

 Pickwell Down Sandstones is therefore typically Upper Devonian. 



II. — The Facial Suture of Trilobites. 



By Professor H. H. Swinnerton, D.Sc, F.G.S., F.Z.S., University College, 



Nottingham. 



Introduction. 

 rPRILOBITES in common with all other Arthropods shed their 

 JL more or less rigid external covering or exoskeleton periodically. 

 To accomplish this ecdysis it is necessary for this covering to split 

 somewhere; and it is highly probable that the facial suture was the 

 line along which such splitting took place. There seems, however, 

 to be a tendency to assume that all lines which served this purpose 

 are homologous. This has introduced unnecessary difficulties into 

 the study of Trilobite classification. The object of this paper is to 

 do something towards the elimination of this source of error. 



The Segmentation of the Head. 



A consideration of the segmentation of the cephalon will do much 

 to give precision to our ideas of the position and homologies of the 

 facial suture. 



Bernard 3 considered that the number of segments in the head 

 region was not fixed, and that within the order Trilobita new 



1 Bothriolepis cristata, B. H. Traquair, Fishes of the Old Bed Sandstone 

 (Mon. Pal. Soc, 1906), p. 130, pi. xxxi. 



2 See B. H. Traquair in Brown & Buckley's Vertebrate Fauna of Moray 

 Basin, 1896, p. 257. 



3 H. M. Bernard, "The Systematic Position of Trilobites": Q.J.G.S., 

 vol. 1, p. 414, 1894. 



