OLENELLUS AND OTHER GENERA OF MESONACID.E 243 



the jMesonacidas of whicli we have the young cephalon. The small 

 first glabellar lobe of Callavia is an illustration of the survival of 

 a primitive character in the adult of a later genus or it may be an 

 instance of reversion to a primitive character. The anterior glabellar 

 lobe of Pccdetimias [pi. 34] and Olenellus [pis. 34-39] is expanded 

 and convex when found in a matrix favorable to preserving the con- 

 vexitv. Most snecimens have been found in shales which accniint<; 

 for the flattening of the lobe and the fracturing of the test not oniy 

 of the lobe but of the adjoining parts of the cephalon. The expan- 

 sion of the anterior lobe of the glabella in the genera Mesonacis, 

 Elliptoccphala, Holniia, JVanneria, Pccdenmias, and Olenellus indi- 

 cates that these genera are of later origin than Nevadia, and this 

 conclusion is strengthened by the evidence afforded by a comparison 

 of the thorax of the genera. Callavia has the primitive glabella, 

 but its thorax indicates a later origin than the genera Nevadia. 

 Mesonacis, and Elliptocephala. 



Another interesting character of the anterior lobe is the presence 

 in 0. logani [pi. 41, fig. 9] of a faint furrow that extends inward a 

 short distance from the point where the anterior margin of the pal- 

 pebral ridges joins the anterior glabellar lobe ; this pair of furrows 

 indicates that the lobe is formed of two segments of the original 

 primitive cephalon.' The palpebral segment is beautifully shown by 

 the young of Elliptocephala asaphoides [pi. 25, figs. 9 and 10]. 



Hypostoma. — ^The hypostoma has a convex central body that is 

 narrowed posteriorly by grooves that separate the body from a trans- 

 verse posterior portion on which maculae may be present. It may 

 be attached directly to the anterior doublure of the cephalon or by 

 a narrow process [pi. 34, figs. 5-7]. Minute specimens of the hy- 

 postoma of Wanneria 'haili [pi. 31, fig. 9] show a perforated, 

 flattened marginal border on the posterior and postero-lateral sides. 

 As the hypostoma increases in size the outer rim disappears and the 

 test between the lobes forms a denticulated margin as in Pccdeuinias 

 transitans [pi. 34, fig. 7]. The next development is the absorption 

 of the thickened points and the formation of true spines [pi. 34, 

 fig. 5] . This type of hypostoma is found in Elliptocephala asaphoides 

 [pi. 24, fig. 8], Holniia kjernlii [pi. 27, fig. 7], Wanneria halli [pi. 

 31, fig. 9], Pccdeumias transitans [pi. 34, figs. 5 and 6], Olenellus 

 gilberti [pi. 36, fig. 5], Olenellus freinonti [pi. 37, figs. 21 and 22], 



^ This anterior pair of furrows is shown for Paradoxides by Barrande's 

 illustrations of P. spinosus [Barrande, 1852, pi. 12, figs. 2, 3, and 6; and pi. 13, 

 fig. i] and P. pusillus [Barrande, 1872, pi. g, fig. 23 1. 



