F. R. Coivper Reed — Blind Trilohites. 501 



TentacuUtes beds of the Continent, In this case, as with the blind 

 Illmii, we must regard the loss of eyes as a secondary acquired 

 feature and an adaptation to certain conditions of life. 



(3) Order Proparia. 



We come now to the highest order of the trilobites, which Beecher 

 has termed the Proparia. It comprises the families Encrinuridee, 

 Calymenidse, Cheiruridge, and Phacopidge, and we find amongst them 

 the blind genera Dindymene, Prosopiscus, Areia, and Placoparia. 



The genus D/?i%?newe (Corda) ^ belongs to the Encrinurid^, which is 

 morphologically the lowest family in the- order. In this genus free 

 cheeks and compound eyes are wanting, and the facial suture is 

 presumably marginal. The head, therefore, in spite of the secondary 

 specialization of the glabella as shown by its loss of segmentation, 

 exhibits the primitive characteristics of the Hypoparia. Probably 

 this genus is phylogenetically as well as morphologically at the base 

 of the family, as is the case with Areia in the Cheirurida3. It is 

 possible that the tubercle on each fixed cheek in some species 

 (e.g. J). Hughesice, Roberts) may be a visual organ similar to the 

 eye-spot in the young Trinudeus, but of this we have no proof. 

 Only two British species are known, i.e. D. Hughesice (Roberts),^ 

 from the Bala beds of Norber Brow, and D. Cordai (Etheridge and 

 Nicholson)^ from the Drummuck beds of the Girvan district. 



The genus Prosopiscus (Salter),* of which little is known, was 

 described as possessing no eyes, and is placed by Beecher amongst 

 the Encrinuridee. 



The genera Areia (Barrande) and Placoparia (Corda) ^ should 

 probably be included amongst the Cheiruridge ; and from a number 

 of sti'uctural features exhibited by these two forms we are 

 led ^ to regard them as early and primitive types of the family 

 in which the ordinary ontogenetic development has been partly 

 arrested, resulting in the retention of some larval characters in 

 combination with secondary modifications. When the loss of visual 

 organs is the result of degeneration or special adaptation, only the 

 head-shield is aifected, as is shown by the blind TllcBni ; the process 

 of degeneration does not affect, as a rule, other parts of the body 

 unless their functions are correlated with the sense of sight. The 

 condition of blind cave-animals, with closely allied species provided 

 with eyes outside the cave, teaches us this fact also. So that we are 

 debarred from considering the larval characters in the general body- 

 structure of Areia and Placoparia as concomitant with or resultant 

 from the absence of eyes. These larval features are not signs of 

 degeneration or adaptation, but are typical of a certain early stage 

 in the evolution of the Cheiruridee. The early stratigraphical horizon 



1 Hawle and Corda, Prodr. Mon. Boh. Trilob., 1847, p. 119. 



2 S. H. Reynolds, Geol. Mag., Dec. IV, Vol. I (1894), p. 108, PI. IV, Figs. 1, 5. 



3 Monogr. Sil. Foss. Girvan, fasc. i (1878), p. 115, pl. riii, fig. 8. 

 * Salter and Blanford, " Palfeont. of Niti," 1865. 



6 Hawle and Corda, Prodr. Mon. Bull. Trilob.. 1847, p. 128. 

 6 Geol. Mag., Dec. IV, Vol. V (1898), p. 206. 



