246 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



species of the Mesonacidse. It is very small, essentially rudimentary, 

 and its segmentation is limited to one transverse ring on the median 

 lobe [pi. 29, fig. 11]. 



The telson of OlcncUus is not considered to be a true pygidium. 

 It resembles the telson of Limnlus, but this resemblance does not 

 necessarily indicate that OleneUiis was the ancestor of Limuhis; its 

 origin does, however, indicate the manner in which the telson of 

 Liuiiilus may have originated. 



DELIMITATION OF GENERA 



The cephalon in all genera of the Mesonacidse is so nearly similar 

 that only specific differences appear to be present, except in Nci'adia 

 and Callavia, which have a small anterior glabellar lobe. The modi- 

 fications of the thorax are largely taken as the basis for generic 

 separation. The pygidium is essentially the same in all of the genera. 



Ncvadia. — Nevadia [pi. 23] is characterized by the presence of 

 small, simple segments (primitive segments) on the posterior portion 

 of the thorax that have not been developed to the same degree as 

 the segments anterior to them. In the type Ncvadia ivccksi the 

 posterior eleven primitive segments have only the axial lobe and a 

 spinose continuation on each side [pi. 23, figs, i, 2, and 4] ; the 

 grooved pleural lobe of the seventeen more specialized anterior seg- 

 ments is not present. The spinose extensions of the posterior seg- 

 ments are proportionally much rounder and smaller than those of 

 the anterior seventeen segments. As far as known the posterior 

 thoracic spines of EUiptocephala [pi. 24, figs, i and 9] and the great 

 spine of the fifteenth segment of Wanneria [pi. 30, fig. 11] and 

 Mesonacis [pi. 26, fig. i] were not developed in Nevadia. 



The only species referred to Ncvadia is A^". zt'ccksi Walcott. 



Mesonacis. — This form [pi. 26, fig. i] is essentially the same as 

 EUiptocephala, but it has an enlarged third segment in the adult and 

 a strong spine on the fifteenth segment. The posterior contracted 

 segments are also of a different character. In Mcsoiuicis they are 

 similar to those anterior to the fifteenth segment, while in EUipto- 

 cephala asaphoides they lose the long-curved pleura so characteristic 

 of the anterior thirteen segments. 



The posterior ten segments may be said to be [pi. 26, figs. i. 2. 

 and 3] less developed, proportionally than the anterior segments, 

 although possessing a well-defined furrowed pleural lobe. A trace 

 of this character is also preserved in Callavia hroggcri [pi. ly, fig. i] 

 where the two posterior thoracic segments are proportionally smaller 

 tlian the anterior segments. 



