OLENELLUS AND OTHER GENERA OF MESONACID.E 297 



Yoiing stages of grozvth. — Nothing is known of the younger stages 

 of growth of the genotype, WA li/alcoftaniis [pi. 30], but of the closely 

 related species, W. halii [pi. 31], there are examples of a number of 

 the younger stages of growth of the cephalon. These show that in 

 the youngest stage of growth known [pi. 31, fig. 8] the form is much 

 like that of the young of Pccdeumias transifans [pi. 32, fig. i, pi. 25, 

 fig. 22], and Elliptoccphala asaphoides [pi. 25, fig. 10]. The most 

 notable changes resulting from increase in size are the diminution in 

 size and length of the palpebral lobes [pi. 31, figs. 8, 7, 4, 2, i, 3], 

 the separation of the genal from the intergenal spines [pi. 31, figs. 

 8, 6, 4, i], and the widening of the glabella back of the first lobe 

 [pi. 31, figs. 8, 5, 7, 4] until its sides are sub-parallel; the change in 

 form of the first or anterior lobe of the glabella is shown by figs. 8, 

 5, 7, 4, 2, I, of pi. 31. Due allowance should be made for the ex- 

 pansion or widening of the anterior convex lobe as the result of flat- 

 tening by compression in the shale. 



Genotype. — Olenellus {Holmia) zvalcottamis Wanner. 



The generic name is given in honor of Prof. Atreus Wanner, of 

 York, Pennsylvania, who first described the type species. 



Stratigraphic range. — Lower Cambrian (Georgian). The geno- 

 type occurs in the York formation, Olenellus zone, in the upper por- 

 tion of the Lower Cambrian terrane, and IV. halli occurs in the same 

 zone in the Montevallo shale. W. gracile is found about 2,000 feet 

 down in the St. Piran formation of the Lower Cambrian of Alberta, 

 Canada, and 1,450 miles to the south in Nevada it occurs 1,200 feet 

 or more below the zone of Olenellus gilberti, which corresponds to 

 about the horizon of Wanneria •halli and W. zualcottanus in Alabama 

 and Pennsylvania, respectively. 



Geographic distribution. — The genotype occurs in an east and west 

 belt across the central parts of York and Lancaster counties, Penn- 

 sylvania. W. halli is found in central Alabama, and W. gracile in 

 Nevada and Alberta, Canada. 



Observations. — The cephalon is similar in generic characters to 

 that of Elliptocephala, Mesonacis, Pcedeumias, Olenellus, and Holmia, 

 but differs from that of Callavia in having a more expanded anterior 

 glabellar lobe, and in not having a large occipital spine. The thorax 

 has seventeen segments of the Callavia broggeri type [pi. 27, fig. i], 

 in that the segments continue of a uniform width out to where the 

 margins converge into a strong backward curving point, but they 

 differ in having a broad pleural furrow of the Olenellus thompsoni 

 type [pi. 35, fig. i], instead of the narrow oblique furrow of C. 

 broggeri. The great spine of the fifteenth segment of the adult 



