OLENELLUS AND OTHER GENERA OF MESONACID.E 347 



I regard Olenelloides as representing a degenerate form of the 

 Mesonacidae that came into existence shortly before the decadence 

 and disappearance of the family. 



Mr. B. N. Peach [1894, p. 668] stated when proposing the name 

 Olenelloides that the name was " intended to show its strong like- 

 ness to some larval stages of other Olenellids." 



Dr. Charles E. Beecher later [1897, p. 191] suggested that this 

 genus might be the young of Olcnellus or some related form. I 

 think, now that we have so much additional information about the 

 young stages of the Mesonacida?, that it may be considered, as just 

 stated, as representing a degenerate genus of the Mesonacidae. 



OLENELLOIDES ARMATUS Peach 



Plate 40, Figs. 2 and 3 



Olenclhis {Olenelloides) armatiis Peach, 1894, Quart. Journ. Geol. Soc. 



London, Vol. 50, pp. 669-670, pi. 32, figs. 1-6. (Described. None of the 



specimens figured are represented in this paper.) 

 Olenelloides armatus Moberg, 1899, Geol. Foren. i Stockholm Forhandl., 



Bd. 21, Hafte 4, pp. 314 and 320, pi. 13, fig. 6. (Mentioned at several 



places in the te.xt. The figure is copied from Peach, 1894, pi. 32, fig. 3.) 



Dr. B. N. Peach has given a very full description of the material 

 illustrating this species that was available when he made his study 

 of it in 1894. Through the courtesy of the Director General of the 

 Geological Surs-ey the specimens studied by Dr. Peach and a num- 

 ber collected since were sent to me by Director Home of the Scot- 

 tish Survey. In the material I found one entire dorsal shield and 

 its matrix. The matrix, being the clearer, is illustrated by fig. 3, pi. 

 40. The following description is drawn from it and the specimen 

 it is the matrix of. 



Dorsal shield small, elongate, converging in outline from the 

 broad cephalon to the narrow pygidium ; moderately convex. 



Cephalon roughly hexagonal with the anterior and posterior sides 

 each equal to the length of two of the shorter right and left sides. 

 The outline is broken by three angles with long spines on each 

 side ; the postero-lateral angle corresponds to the intergenal angle 

 of the young cephalon of Olenellus gilherti [pi. 36, figs. 11-14] ; 

 the median angles and spines correspond to the genal angles of the 

 same, and the antero-lateral angles and spines to similar angles 

 and spines of the young cephalon of O. gilherti; the antero-lateral 

 spines are located at the points where the facial suture would ap- 

 parently terminate on the outer margin. The round marginal rim 



