F. R. Cowper Reed — Notes on Trinucleus. 349 



Sedgwick Museum JSTotes. 

 lY. — ^N'oTEs ox THE genus Trinucleus. — Part III.* 



By F. K. Cowper Keed, Sc.D., F.G.S. 



(PLATES XXVIII AND XXIX.) 



I. The Glabella. 



1. llie Lohation. 



rpHE characters of the lateral furrows of the glabella and of the 

 1 pits which occur iu the axial furrows of the head-shield have 

 not had sufficient attention paid to tliem, and a comparative study 

 of their development in various British species reveals some 

 interesting and important features. 



It has been commonly assumed that lateral furrows are absent 

 in some species, and at any rate the omission, of any notice of 

 their occurrence in certain cases is difficult to understand. 

 Frequently, moreover, it has been stated that only two pairs 

 are present, even in those species in which the presence of 

 any lateral furrows has been recognized. The nature of the 

 so-called stalk or neck of the glabella has been also generally 

 misunderstood. McCoy's" division of the genus Trinucleus into 

 two genera, Trinucleus, sens, restr., and Tretaspis, was largely based 

 on the presence or absence of this stalk and of lateral furrows ; for 

 he says the furrows at the base of the glabella distinguish Tretaspis 

 from Trimicleus, the latter being stated to be without lateral furrows, 

 though the presence of the ' eye-line ' on the cheek was chosen as 

 the primary distinctive feature of the former. Nicholson and 

 Etheridge,^ while rejecting Tretaspis as a sub-generic division, 

 mention an additional third pair of furrows at the sides of the 

 anterior swollen portion of the glabella in Trimuleus Bucklandi, 

 though they do not seem to regard them as segmental furrows. 



In the recent new edition (1913) of the Eastman-Zittel Text- 

 look of PalcBontology the old genus Trinucleus is split into three 

 genera, of which two {^Trinucleus, sens, restr., and Tretaspis, McCoy) 

 are said to possess two pairs of lateral furrows on the glabella. 

 Additional confusion is thus introduced into the usage of the 

 terms. In the case of the third genus, Cryptolithus, Green, of which 

 Or. Goldfussi (Barr.) is figured as an example, no mention is made of 

 any furrows or pits and none are shown in the figure. Ruedemann,'' 

 however, recognized three pairs of lateral pits in Tretaspis reticu- 

 latus. Rued. 



The general failure to recognize more than two pairs of segmental 

 furrows or pits on the glabella is undoubtedly due to the com- 

 paratively weak development or obsolescence of the first or anterior 



^ Parts I and II were published in 1912 (Geol. Mag., Dec. V, Vol. IX, 

 pp. 346-53, 385-94). 



■^ McCoy, Syn. Palaeoz. Foss. Woodw. Mus., 1855, pp. 144-7. 



^ Nicholson & Etheridge, Men. Silur. Foss. Girvan, fasc. ii, pp. 190-7, 1880. 



* Kuedemann, Bull. 49 New York State Mus., Palffiont. Papers 2, 1901, 

 p. 41, pi. iii, figs. 11, 15-20. 



