134 NEW YORK STATE MUSEUM 



several trilobites. This is especially probable in the case of hollow, 

 sometimes curved spines on top of the glabella, such as Ampyx 

 possesses, and where the animals led a mud-groveling life. 



2 The eye tubercles are always situated at the very highest spot 

 of the carapace, either on the apex of the bulging frontal lobe of 

 the glabella as in Cryptolithus (Trinucleus), or as in Isotelus and 

 Asaphus where the glabella abruptly bends downward, on the 

 prominent posterior portion between the last lobes. It may have 

 wandered forward and backward, but it is always upon the highest 

 point of the glabella which would be the place of advantage for a 

 median eye. It completely agrees in this regard with the position 

 of the ocelli in the eurypterids. 



3 The tubercle is generally situated between the posterior por- 

 tions of the lateral or compound eyes, as in the typical eurypterids. 

 This position seems to us best explained by the fact that the 

 median eye is according to its origin always nearest to the brain, 

 which is, in the phyllopods and most other crustaceans, situated in 

 the dorsal region of the head, beneath or between the lateral eyes. 

 The median eye is typically, as in Apus (the phyllopod that is 

 always referred to in discussions of the taxonomic position of the 

 trilobites), "in immediate contact with the brain." 1 We see for 

 this reason, in the prevailing position of the tubercle between the 

 posterior portion of the lateral eyes, a strong argument for its 

 ocular character. 



4 In this connectioon the observation is also significant that the 

 tubercle is in many cases found at the posterior extremity of a dis- 

 tinct crest, extending backward a short distance upon the glabella. 

 A like crest bearing the ocelli of the median eye, has been observed 

 in Stylonurus excelsior by Hall and Clarke (no. 7, 

 pi. 26) and it occurs in other eurypterids. Species which show this 

 feature very well marked are Asaphus raniceps (see 

 Schmidt, no. 27. Lief. 2, pi. 2, fig. 2), Nile us armadillo 

 (op. cit., Lief. 3, pi. 8, fig. 13), 111 a en us (Bumastus) 

 barriensis (op. cit., Lief. 4, pi. 27, fig. 1). It is probable that 

 this crest is an analogon of the " eye line " of the primitive trilo- 

 bites and, at least in part, marks the path of the nerve leading to the 

 median eye. 



Besides the structural features mentioned so far as pointing to 

 an ocular function of the median tubercle, we believe that also the 

 following general considerations support this view. 



1 Parker & Haswell, no. 14, p. 536. 



