104 



Professor H. H. Swinnerton — 



segments had been added to the posterior margin. He based this 

 view upon two considerations : first, that the number of segments 

 indicated on the axial portion of the dorsal side of the cephalon 

 varied; second, that the last or occipital segment bears so close a 

 resemblance to a trunk segment as to suggest that it had "been 

 recently incorporated". 



In discussing the former consideration, he observes that whilst 

 the majority of trilobites show traces of five segments on the 

 glabella, some, e.g. Microdiscus and Triarthrus, show only four. 

 This line of reasoning is fallacious, for glabellar furrows are often 

 reduced or smoothed out, so that if indications of four segments are 

 left it is not safe to say that there are not more than four segments in 

 the head, but it is safe to say that there are at least four. The 

 presence of five segments on the glabella of JEodiscus speciosus 1 

 and the discovery since his day of five pairs of cephalic limbs in 

 Triarthrus refute his first argument. 



a b 



Figures showing the position of the "facial suture" of Raymond and 

 (?) Beecher in Pcedeumias and Agnostus. a. Head-shield of Padeumias 

 transitans (Walcott) after Walcott (Smiths. Misc. Coll., vol. liii, 

 pi. xxxiv, fig. 6). b. Carapace of Agnostus nudus (Beyrich) copied 

 from Raymond (Amer. Journ. Sci., 1917, fig. 1). d. Doublure of 

 PcBdeumias reflected from underneath the head-shield, s.m. Sutural 

 margin of the doublure, s. Approximate position of the marginal suture 

 (facial suture of Raymond) on ventral surface, e. Eye. c. Cephalon of 

 Agnostus according to authors, p. Cephalon of Agnostus according to 

 Raymond, m.s. Marginal suture ("ventral facial suture" of Raymond 

 and ?Beecher) of Agnostus. f.c. Ventral free cheeks according to 

 Raymond and (?) Beecher. 



With regard to the resemblance of the occipital segment to the 

 trunk segments, it may be observed that this is merely an illustration 

 of the principle that in segmented animals specialization of the 

 posterior lags behind that of the anterior segments. Thus, within 

 the Trilobita the glabellar furrows are obscure or absent most 

 frequently in front, and again, it is this part of the glabella that 

 varies most in shape and dimensions; compare for example, such 

 forms as Conocoryphe, Olenus, Paradoxides, Phacops, Staurocephalus. 



1 Bull. U.S. Geol. Surv., No. 30, p. 154, 1886. 



