4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



these segments have pleurae, as ilkistratecl in the figures of M. 

 vermontana (see particularly Walcott, 1910, pi. 26). According to 

 my present views, Olcncllus had such segments, perhaps fewer in 

 number, but zmthoiit pleurae, i. c, without dorsal furrows. These are 

 illustrated in the specimen figured by Walcott in 1910, on plate ZZ^ 

 figure I, as Pacdumias transitans. It must be remembered that most 

 of our ideas of Olcnellus are based not on Hall's original figures of 

 O. thompsoni but on the incorrect restoration, made from a very 

 poorly preserved specimen, first published by Walcott in 1886 and 

 subsequently widely copied. This figure, as Walcott stated in 1910, 

 ( description of pi. 35, fig. i ) is incorrect in representing the anterior 

 lobe of the glabella as not reaching the rim. In this respect OlencUus 

 and Mcsonacis are identical. 



The peculiar habit possessed by most of the jVIesonacidae, that of 

 the genal spines advancing forward, is not an individual character- 

 istic, as is commonly assumed, but is specific. This is clearly indicated 

 in the following specific grouping of the specimens from this locality. 

 If the position of the genal spines were a matter of individuality, it 

 would not be possible, as stated later in the descriptions, to assemble 

 a dozen or more specimens into each of several species in which there 

 is no variation in this respect. 



PAEDUMIAS Walcott, 19 10 



Pacdiiniias Walcott, 1910, Smithsonian Misc. Coll., Vol. 53, No. 6, p. 304. 

 (Described as a new genus.) 



The original description of the single species referred to this genus 

 in 1 910 is based mainly on the specimens from York, Pennsylvania, 

 but it is clearly stated that the type locality is in X'ermont. The 

 observations on which this original discussion were based were made 

 on some specimens that cannot belong to the genus. The best example 

 so used is the large, well preserved specimen illustrated by Walcott, 

 1910, as figure i on plate 33, which I now refer to Olcncllus. Walcott 

 states on page 308, " Nearly all the specimens of Pacdumias found at 

 York have the typical cephalon of P. transitans, as shown on pi. 34, 

 figs. 2-4. In all of these the anterior lobe of the glabella is some 

 distance from the frontal rim of the head, while in typical Olcncllus 

 thompsoni and Mcsonacis vermontana from Vermont the anterior lobe 

 touches the frontal rim." Thus it will be seen that he had in mind 

 what he calls the " elongate form " as the typical form of Pacdumias. 

 Accordingly I have chosen the specimen figured on plate 34, figure i 

 as the lectotype (U. S. Nat. Mus., Cat. No. 56808). 



