[MATTHEW] STUDIES ON CAMBRIAN FAUNAS 79 



of strenua, because it has a wide fore-limb to the cheeks, a smaller 

 glabella, and a weaker occipital ring. It is undoubtedly nearer to Shaler 

 and Foerste's F. Attleborensis, than to any other described species. 



MiCMACCA Walcotti, n. sp. (PI. iV., fig. ba to d.) 

 Solenopleura boynbifrons (Matt.) Wale, Fauna of Olenellus Zone, p. 686. 



This species is of common occurrence among those of the shale above 

 the conglomerate at Manuel's. Mr. Walcott has figured it on PI. xcviii., 

 fig. 5, 5a and b of his Olenellus Fauna. He has referred it to my Soleno- 

 pleura (?) bombifrons, stating that it is the adult form of that species. A 

 comparison of the two, however, will show that there are several objec- 

 tions to this view. The eye-lobe of S. (?) bombifrons is not continuous as 

 is that of M. Walcotti and is also shorter. This is a very unusual con- 

 dition of development in a trilobitc, I know of no instance of the lengthen- 

 ing of the eye-lobe during growth, but many in which it becomes shorter. 

 The posterior margin of S. (?) bombifrons differs in the absence of inter- 

 ocular spines ; and the posterior extension of the suture runs outward, 

 much further than in M. Walcotti. 



The actual larval heads of this species have narrower glabellas than 

 the adult and more distinct glabellar furrows, and the eye-lobes are 

 continuous to the posterior furrow. 



The following are the specific characters of M. Walcotti : Head- 

 shield sub-semicircular, and strongly arched. Middlo-piece sub-quadrate. 

 Front margin with distinct fold. Glabella prominent, reaching almost or 

 quite to the fold, cj^lindrical, broadly rounded in front, marked by two 

 shallow indistinct furrows, furrows directed backward, the anterior often 

 not traceable ; width of glabella about three-fifths of its length. Occi- 

 pital ring wide, divided from the glabella by a shallow furrow, which is 

 most distinct in the outer third ; the ring carries a spine which is as long 

 as half of the width of the glabella. The fixed cheek lies much below 

 the level of the glabella, is moderately arched, and with the eye-lobe is 

 two thirds of the width of the glabella. The ocular fillet is prominent, 

 and with the eye-lobe forms a continuous ridge from the glabella to the 

 posterior marginal furrow. The posterior marginal furrow is strong and 

 widens greatly in going outward ; the posterior marginal fold is sharply 

 upturned, geniculated in the middle, and near the dorsal suture bears an 

 interocular spine, directed backward. The dorsal suture scarcely arches 

 outward either before or behind the eye-lobe. 



The movable cheek is moderately arched on the outside, and bears a 

 spine about as long as two-thirds of its length from the front to the 

 posterior border ; the area and fold are of about equal width, and the 

 former, opposite the eye-lobe, is bent upward at a right angle to the latter. 



