170 F, R. C. Reed—On the genus Trinucleus. 
Lake,! in regarding the genus Orometopus as the earliest genus of 
the Trinucleide, concluded that it was not improbable that the ocelli 
in Zrinucleus represented normal compound eyes in a degenerate 
condition. Swinnerton? is also inclined to regard the absence of 
normal facial sutures and eyes not as a sign of early phylogenetic 
position but as secondary modifications of the Opisthoparian type 
of head-shield. For he does not consider the marginal suture as 
homologous with the facial sutures of the Opisthoparia and Proparia. 
These views are diametrically opposed to those put forward by 
Beecher,’ on which the classification of the trilobites has of late years 
been based. 
It may be pointed out that the present author, in 1898,* drew 
attention to the importance of adaptive changes in trilobites in 
connection with the loss of eyes and to the modifications of the head- 
shield which accompanied this loss, and it was maintained that 
blindness was not by itself any evidence of primitive phylogenetic 
position or of reversion to early stages of development. In the case 
of Zrinucleus blindness was considered to be an adaptation to special 
environment. Dollo® has treated this subject at considerable length, 
and has concluded that 7. concentricus lived in the mud under aphotic 
conditions as a member of the benthos. 
1. Facial Sutures and Genal Areas. 
The controversy as to the presence or absence of facial sutures 
crossing the genal areas (apart from the marginal suture round the 
fringe) has been waged by many paleontologists.© But no satisfactory 
evidence of the occurrence of such sutures in young or adult has been 
produced, and McCoy’s’ figures and description of their presence in 
his genus Zretaspis are not supported by an examination of the 
specimens which he used. It is needless to recapitulate here the 
opinions of the various writers on the subject, for in the case of 
the English species here studied it has been possible for me to test 
the accuracy of their observations and conclusions. The line of 
‘inquiry which they followed was not often comparative, and the 
principles of development were not understood by the earlier 
investigators. The researches of Beecher and other American 
workers have thrown a flood of light on the ontogeny and phylogeny 
of the trilobites, and from the adoption of the principles which they 
have established the position of Zrinucleus has been regarded as fairly 
secure amongst the most primitive group, Hypoparia. But recently, 
as above remarked, signs of dissatisfaction with this conclusion have 
been apparent. 
If we regard the genus as now possessing degenerate characters 
which it has acquired by the loss of certain structures, such as facial 
1 Lake, Cambrian Trilobites (Paleont. Soc.), 1907, p. 45. 
2 Swinnerton, GEOL. MaG., Dec. VI, Vol. II, p. 489, 1915. 
° Beecher, Amer. Journ. Sci., ser. Iv, vol. iii, pp. 89-106, 181-207, 1897. 
4 Reed, GEOL. MaG., Dec. IV, Vol. V, pp. 489-47, 552-9, 1898. 
> Dollo, Bull. Soc. Géol. Belgique, xxiii, p. 417, 1909. 
® Woods, article on Trilobita in The Cambridge Natural History, vol. iv, 
Crustacea, pp. 226, 230-1, 244-5, 1909. 
* McCoy, Syn. Brit. Pal. Foss. Woodw. Mus., p. 146. 
