502 F. R. Cou-per Beed— Blind Trihhites. 



on which these two genera occur (i.e. Areia on Dd 1 and T)d 5 in 

 Bohemia ; Plncoparia in J)d 1 and Dd 2 in Bohemia, and in the 

 Llanvirn of Wales, etc.), and their disappearance before the family 

 reached its maximum of differentiation, point to the same conclusion. 



None of the Calymenidee are analogous to these primitive 

 Cheiruridai, and none are without eyes. 



Amongst the Phacopidee, one species of tlie type genus, Fhacops, is 

 supposed to be without eyes, though possessing the features of one 

 of the most differentiated and latest groups of the genus. This 

 species is Ph. (Trimerocephalus) Icsvis (Miinster), and it occurs in the 

 Upper Devonian of England. Salter,^ while describing the species 

 as devoid of eyes, says that ver}^ probably their apparent absence is 

 only due to the imperfect preservation of the specimens. McCoy, ^ 

 however, takes the absence of eyes as one of the characteristics of 

 the subgenus. Barrande^ reviews the historj' of this form in some 

 detail, and accej)ts the view that it is blind. If this be indeed the 

 case, it must be analogous to the blind species of Illcenus, and of no 

 jDhylogenetic or ontogenetic significance. For the degeneration of 

 the visual organs we must again seek some pathological or external 

 cause. 



Doubtfid Genera. 



We have now been through all the families of ihe Trilobita, and 

 diycussed their blind forms, but there still remain a few blind 

 genera the position of which is doubtful. We may first mention 

 the blind genus TypMoniscus (Salter),* from the Lower Devonian 

 of South Africa, which was assigned by Salter to the Cheiruridee. 

 Another genus whose affinities are uncertain is Cyphoniscus (Salter).^ 

 Only one species (C. socialis, Salter) is known, and it occurs in the 

 Upper Bala Limestone of the Chair of Kildare and in the Keisley 

 Limestone. It possesses a facial suture, but no eyes have been 

 observed, and the free cheeks themselves have not so far been 

 discovered. Salter'' was of the opinion that eyes were present 

 because there were probably separable fi-ee cheeks, but as it is now 

 definitely ascertained that there are many blind species which have 

 marginal free cheeks of a narrow bandJike form cut off by a simple 

 facial suture, as in this species, it appears probable that Cyphoniscus 

 was also blind. 



Ttresias (McCoy),' first described from the Chair of Kildare Bala 

 Limestone, but recentl}^ also from the Keisley Limestone,® is destitute 

 of eyes and appears to be allied to Ampyx, but only the head-shield 

 is known. Isocolus (Angelin)/ of which the type is I. Sjogreni 



1 Mon. Brit. Trilob., 1864, p. 16. 

 ' McCoy, Syn. Brit. Pal. Foss., 1851. 

 ^ Barrande, Syst. Sil. Boh., SuppL, vol. i, p. 158. 



* Geol. Trans., 2nd ser., vol. vii (1846), p. 221, pi. xxv, fig. 14. Mon. Brit. 

 Trilob., note on p. 60. 



s Proc. Brit. Assoc, 1852, p. 60. 



^ Mem. Geol. Surv. Org. Rem., dec. vii, pi. ix. 



' Syn. Sil. Foss. Ireland, 1846, p. 43, pi. iv, fig. 1. 



8 Eeed, Quart. Journ. Geol. Soc, vol. lii (1896), p. 410. 



» Pal. Scand., p. 58, pi. xxxiii, fig. 8. 



