F. R. Coioper Reed — Blind Trilohites. 443 



further tlian that the above evidence from the trilohites (and much 

 more that might be adduced) is completely in harmony with them. 



We find in the trilohites a group with an uniform generalized 

 structure and with a singularly complete geological history, and in 

 addition to this we possess a knowledge of the ontogeny of all the 

 principal families of different geological periods, and it is found to 

 correspond with the phylogeny and geological succession of the 

 members of the group. We can therefore proceed with a con- 

 siderable amount of confidence in determining what are true larval 

 and primitive characters, and ija separating from them those which 

 are of the nature of adaptations or the result of degeneration. 



Characteristics of Blind Trilohites. 



As may be gathered from the above remarks, it will be found that 

 the characteristics of a large proportion of blind trilohites will be 

 those of the earlier, lower, and more primitive families or genera 

 which possess no compound eyes, because of their phylogenetio 

 position. Those of this rank which possess organs of vision do so, 

 not by virtue of their stage of evolution, but by reason of secondary 

 specialization ; and these ' eye-spots ' with which they are furnished 

 are not homologous with the compound eyes of the higher forms. 

 In anticipation of much which is to follow we may remark that the 

 blind higher forms, on the other hand, which have their phylogenetio 

 position determined by their close structural resemblance to and 

 association with forms possessing compound eyes, so that their 

 affinities admit of no doubt, are characterized as a rule by the 

 degeneration of the free cheek into a mere marginal band. This 

 feature appears to indicate that the development of the compound eye 

 and free cheek is in some way interdependent, for we find no blind 

 form of high rank with a large triangular free cheek ; the free cheek 

 is always narrow ; and the lower blind forms, as we have seen, have 

 either no free cheek or else a very narrow marginal one. This 

 reduction of the size of the free cheek is accompanied by the 

 migration outwards of the facial suture and its simplification ; and 

 we remember that the marginal and simple course of the facial 

 suture was a primitive feature. Thus the loss of the compound 

 eyes in the higher forms is accompanied by a certain amount of 

 reversion to primitive conditions in the head-shield. The other 

 parts of the body are, so far as we know, unaffected, but we may 

 reasonably assume that some modifications in the appendages took 

 place in conjunction with the loss of eyesight, as we find to be the 

 case in most of those modern blind Crustacea which are allied to 

 species or genera possessing eyes. Details of each individual genus 

 oi blind trilohites are given below. 



The Blind Genera. 



The most natural system of classification which has so far been 

 proposed for the trilohites is that given by Beecher.^ It has a close 

 correspondence with that drawn up by Salter, and, being based oa 



1 Beecher, Amer. Joui-n. Sci., vol. iii (1897), p. 183. 



