12 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



Harpes, ranging from the Lower Silurian Lower Red Orthoceratite Limestone into the 

 middle Devonian beds. 



It presents itself in the inöst variable shapes, and as it in fact can be followed through 

 its development in the oldest knöwn species, it is snitable to begin its description together 

 with the characteristics peculiar to the oldest or second group in this large snbdivision, 

 that of the Olenellidce. 



Group 1. The Olenellidse. 



In this group we have two families of different age, the older, less developed the 

 Olenellidse proper, and the younger the Paradoxida?. The former consists of the genera 

 Olenellus, Holmia, Mesonacis and Schmidtia, and we shall attend to them first. They 

 have no facial suture 1 and consequently a tripartite head shield like the Arehaie 

 ones, no eyes, but there is that strongly developed and most characteristic facial ridge. 

 As seen in Olenellus Thompsoni 2 the crescent like ridge starts as a direct outflow from 

 the base of the first segment of the glabella, and is in direct continuation with it as an 

 integral part. It is ronndbacked, regularly curved and at its starting point as broad as one 

 of the posterior glabellar segments. It is regularly semilunate, tapers posteriorly and ends 

 near the occipital segment. From the narrow second segment of the glabella, inside the just 

 described larger ridge, a smaller ridge emanates, broader and flatter than the former, slightly 

 curved, and ends between the third and fourth glabellar segment. Already in Holmia Kje- 

 rulfi the ridge is modified. There it is only one ridge, the anterior one, nearly as doubled 

 through a shallow groove running along its back. The second one has dwindled awaj 

 so as to be seen only as a narrow stripe near the occipital segment and ending outside 

 this in a point. The anterior ridge, represented by Holm as consisting of two nearly 

 independant parts, is indeed in one piece, though its dorsal groove sometimes is deep, 

 and it is with its total breadth joint to the first glabellar segment. Along its outer edge, 

 where it lies close to the cheek, a narrow slit runs, and I suppose that it is to be con- 

 sidered ;is the first indication of the forming of ;i facial suture, which however does not 

 reach longer than the ridge. As in Paradoxides it is so tight to the cheek, that there 

 has been no place for an eye. It is no accidental break, its edges being too regular and 

 unbroken in all specimens. This ridge is by far much shorter than in Olenellus and 

 terminates opposite the third segment. 





1 Bekcher Natural Classificatiou of the Trilobites, p. 191, pretends that in Olenellus and Holmia real 

 sutures in a condition of symphysis» occur. He seems lo denie the facial sutures and to accept as »real 

 sutures» the »internal sutures» described hy Holm in Olenellus Kjerulfi. It is highly doubtful if these interiör 

 elevated lines are to lie regarded as sutures. Thej are indeed no sutures, but in reality elevated linear ridges, 

 inclosing, as it were, narrow canals. The real sutures known, the facial sutures, never form elevated lines, be 

 it un the Outside of the head or on the inside. 1'robahly these lines are derived from some now unknown in- 

 teriör organization and it may he fit to remind of the somewhat similar though more numcrous linear canals 



of the branchii i the interiör surface of the great head shield of Ainvs. (Zaddacji De Apodie cancriformis 



Anatome . . . pl. Il ti^. 1) or what HUXLEY (Anatomy Invertehr. Animals p. 281) calls the convoluted »shell 

 gland in the carapace. A quite different strueture is what I suppose to he the incipient facial suture in Holmia. 



2 WALCOTT in lOtli Ann. Kept. U. S. Geol. Survey, pl. 82 f. 2. 



