OLENELLUS AND OTHER GENERA OF MESONACID.E 247 



The species referred to Mcsojiacis are: M. verniontana (Hall), 

 M. inickuntsi (Schmidt), and M. torrelli (Moberg). 



Elliptocephala. — In ElHptoccphala [pi. 24, figs, i, 2, and 9] the 

 posterior five segments are more highly developed than the primitive 

 segments of Nevadia, but not as much so as the segments anterior 

 to them. The abrupt narrowing of the thorax of Elliptocephala is 

 of the same type as that shown by Mesonacis [pi. 26, fig. i], but it 

 does not have the large third segment in the adult stage or the great 

 Spine on the fifteenth segment. 



The one species referred to Elliptocephala is E. asaphoidcs 

 Emmons. 



Callavia. — Callaria [pi. 27, fig. i ; pi. 28, figs. 4 and 8] has a trace 

 of the constricted pleurce of the posterior portion of the thorax in 

 the two last segments ; these are of the same type as the anterior 

 segments. The broad thorax of Callavia with the falcate extensions 

 of the pleurse are quite unlike the narrow thorax of Holmia [pi. 27, 

 fig. 7] with its spinose pleural extensions. There are differences of 

 importance in the cephalon as compared with Holmia. The glabella 

 of Callavia is narrower and more primitive and its intergenal spine 

 is less primitive. The pleural furrow of Callavia is narrow and 

 oblique like that of Parad oxides, while the pleural furrow of Holmia 

 and IVaimeria [pi. 30] is broad and straight like that of all other 

 known genera of the Alesonacid.e. It is doubtful if Callavia should 

 precede Holmia, but from its primitive glabella and the retaining of 

 two shortened thoracic segments it appears to be nearer Elliptoceph- 

 ala than does Holmia. 



The species referred- to Callazna are : C. bicensis Walcott, C. 

 broggeri (Walcott), C. burri Walcott, C. callavei (Lapworth), C. 

 eartlandi (Raw), C. crosbyi Walcott, and C. nevadensis Walcott. 



Elolmia. — Holmia [pi. 27, fig. 7] with its uniform series of seg- 

 ments and simple plate-like pygidium appears to represent a stage 

 in the development of the Mesonacidae that followed the Ellipto- 

 cc phala-M esonacis stages. It has lost the rudimentary thoracic seg- 

 ments of Nevadia, it is without the large third segment of the adult 

 Olenellus and Mesonacis, and it is not known to have had an en- 

 larged third segment at any stage of growth. The posterior segments 

 do not show the restricted character of those posterior to the fif- 

 teenth spine bearing segment of Mesonacis, or the rudimentary form 

 of the oosterior see"nients of Pccdcuniias. 



The species referred to Holmia are: H. kjerulfi (Linnarsson"), 

 H. lundgreni (Moberg), and H. rotvei Walcott. 



