500 F. B. Cotrper Reed — Blind Trilohites. 



agree in all other respects witli those Illcsni possessing eyes and the 

 normal llleenicl structure of the head, we must conclude that they 

 have suffered degeneration of their visual organs, and that it has 

 been accompanied by a reassumption of those primitive features in 

 the free cheeks and facial sutures which are met with in the blind 

 ConocoryphidEe. What has led to the loss of eyesight we will 

 consider later, but we see that we must at any rate regard their 

 condition as a modification of the normal organization of the genus, 

 and not as phylogenetically significant. In other words, we have 

 here to deal with a special adaptive character in contradistinction to 

 one marking a stage in the regular evolution of the group. Similar 

 reversionary or degenerate types in a highly specialized genus or in 

 a family of high morphological rank are occasionally found amongst 

 modern Crustacea when surrounded by abnormal conditions in which 

 these organs of special sense are not required. 



The blind species of IU(i:nus are the following : — 



I. leptopleura (Linnarsson),^ from the Trinucleus beds of Sweden. 



J. Angeliui (Holm)," from the same beds and locality. 



I. ccecus (Holm), =* from the Lyckholm Beds (Stage Fl) of Eussia, 



the Keisley Limestone of England* and the Kildare Limestone 



of the Chair of Kildare.® 



1. galeatus (Eeed),^ from the Keisley Limestone. 



2. aratus (Barrande),'' from Etage T)d 1, Bohemia. 

 J. Katzeri (Barrande),® from Etage Ddl, Bohemia. 

 I. Zeidleri (Barrande),^ from Etage Dd 5, Bohemia. 



It is noticeable that the blind forms in Sweden and Bohemia occur 

 in fine argillaceous beds, while in Eussia and the British Isles they 

 are found in limestone. This is a significant fact, as it indicates that 

 they lived under entirely different physical conditions in the several 

 areas, and that there is a possibility that different causes have led to 

 the same modification. 



In the family Proetidas no blind genus is known, but in two species 

 of Proetus — P. dormitans (Eichter) and F. expansits (Eichter)^" — no 

 eyes have been found. None of the Bronteidse or Lichadidae are 

 known to be blind, but amongst the Acidaspidje there is one blind 

 species of Acidaspis, A. my ops (Eichter), ^^ which occurs in the 



I Holm, " De Svensk. Art. Trilobitslag. IlliBims " : Bih. t. k. Vet. Akad. Handl., 

 Ed. vii (1882), No. 3, p. 118, t. iv, f. 28 ; t. vi, f. 11. 



3 Holm, ibid., p. 120, t. iv, f. 29. 



* Holm, " Eev. d. ostbalt. Silixr. Trilob.," Abth. iii, Illffiniden: Mem. Acad. Imp, 

 Sci. St. Petersb., ser. tii, t. xxxiii (1886), No. 8, p. 162, t. xi, f. lla-d. 



* Quart. Journ. Gaol. Soc, vol. Iii (1896), p. 413. 

 » Ibid., p. 593. 



6 Ibid., p. 414. 



' Syst. Sil. Boh., Suppl., vol. i, p. 68, pl. xiv, figs. 43, 44. 



8 Bull. Soc. Geol. France, a-o1. xiii (1856), p. 532. Syst. Sil. Boh., Suppl., vol. i, 

 p. 72, pl. V, figs. 28-37 ; pl. ti, figs. 1-4 ; pl. xiv, fig. 36. 



9 Syst. SU. Boh., Suppl., vol. i, p. 74, pl. iii, figs. 20-29. 



10 Eichter, Zeitsch. f. Deutsch. Geol. Gesell., vol. xv (1863), p. 659 ; ibid., 

 vol. xvii (1865), p. 361. 



II Eichter, op. cit., vol. xv (1863), p. 670. 



