1847.] SALTER ON TRINUCLEUS. 251 



Fort with tlie main land^ the mud of Haslar Creek would be covered 

 with shingle in a very similar manner. Above the shingle bed is 

 another layer of clay (5) forming the bed of the present estuary. 

 I shall not trouble the Society with any observations which the study 

 of such phsenomena as I have now described might suggest, as this 

 subject has been so frequently and so recently discussed. 



March 10, 1847. 



The following communications were read : — 



1 . On the Structure of Trinucleus, with Remarks on the Species, 

 By J. W. Salter, F.G.S., A.L.S. 



Having met with some unexpected results during a study of the 

 Trilobites, more especially with regard to the genus Trinucleus, so 

 eminently characteristic of the Lower Silurian and Cambrian deposits, 

 I wish to offer a few observations on some changes now necessary in 

 nomenclature, and also on the structural peculiarities arrived at. 



It is to classical feeling we owe the name of this genus, for Llhwyd's 

 Trinucleus, published in the ' Philosophical Transactions ' so far back 

 as 1698, meant no more than the general term Trilobite, and could 

 not, except by courtesy, set aside the name Cryptolithus, first pro- 

 posed, with characters, by Green. Sir R. I. Murchison has revived 

 the old and far more expressive name, and all subsequent European 

 naturalists have adopted it. 



The genus can hardly be confounded with any other — the peculiar 

 perforated border and the small number of segments in the body, 

 together forming an animal dissimilar from all related genera. Thus 

 the genus Harpes, which has a broad punctate border, has numerous 

 body-segments and a minute tail ; while Ampyx, another close ally, 

 though similar in the body, wants entirely the ornamented margin. The 

 absence of eyes in the most common species has been supposed generic, 

 and it may be so, for we only have a prominent tubercle in their place 

 in the Tr. seticornis, which probably has only minute scattered lenses 

 upon it ; at all events vision seems not to be essential to this curiously 

 constructed animal. Ampyx is equally destitute. 



The facial suture, — stated by Loven to surround the whole head, — 

 has been detected by Emmerich and myself in its normal position : 

 it may be seen in good specimens, but only on the under surface of 

 the head, and not continuously. Its course is obliquely upwards 

 from the eye tubercle (in one species) to the upper end of the glabella, 

 where it appears to terminate in a solitary deep perforation, similar 

 to those which surround the head (fig. \, a a to b b). 



The glabella, in good specimens, has an obscure furrow at the 

 base on each side, independent of the conspicuous one which separates 



