306 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Stage and the typical adult of O. thonipsoni, that include characters 

 of the adult forms of Hulniia, Mcsonacis, and OlcncUus, also that a 

 form otherwise identical with O. thompsoni has rudimentary thoracic 

 segments and a Holmia-Yike pygidium posterior to the fifteenth spine- 

 bearing segment of the thorax. For this form the name PcrdeiDiiias 

 transitaiis is proposed. In my first notes I referred these forms to 

 Mesonacis, but with better material it became evident that the rudi- 

 mentary segments of P. fransifans were quite unlike those of Mcso- 

 nacis [compare fig. 12, pi. 24, with figs. 2 and 3 on pi. 26]. 



In many specimens of P. transitaiis from York two to six rudi- 

 mentary segments and a small, plate-like pygidium occur beneath 

 and posterior to the fifteenth telson-bearing segment. The rudi- 

 mentary segments are very thin, without pleural lobes, and marked 

 by a broad, simple, transverse furrow ; the ends terminate abruptly 

 with a very short spine at the posterior angle in some specimens. 



The York specimens [pi. 33, figs. 2-5] are similar to those from 

 Vermont [fig. i. pi. 33, and fig. i, pi. 34]. In the typical form of 

 Mcsonacis I'crmontana [pi. 26, figs, i and 3] there are well-defined 

 pleural lobes back of the fifteenth segment of the thorax, and the 

 spine on the fifteenth segment is a characteristic dorsal spine and not 

 a terminal telson like that of O. thompsoni [pi. 34, fig. 9; pi. 35, 

 fig. i]. The spine of fig. i, pi. 33, is nearing the last stage of the 

 change from the Mcsonacis-WkQ dorsal spine to the telson of Olcncl- 

 his. A similar specimen to this was found by Mr. Noah L. Getz one 

 mile north of Rohrerstown, Lancaster County, Pennsylvania. 



Restricting Mcsonacis to those forms in which the segments pos- 

 terior to the spine-bearing fifteenth segment are normal thoracic 

 segments, such as represented by figs, i, 2, and 3, pi. 26, we then 

 refer all with the short rudimentary segments posterior to the spine- 

 bearing fiftt^enth segment to the Pccdcuinias stage of development of 

 the Mesonacid?e as Pccdcuniias transitans; this species includes not 

 only the York specimens, but the large Vermont specimens repre- 

 sented by fig. I, pi. 33, and fig. i, pi. 34. 



In two specimens collected by Professor Wanner the telson has 

 broken away from its base so as to show the tmion of the rudi- 

 mentary segn:ent and the fifteenth segment [see pi. 33, figs. 2 and 

 5]. The telson was hollow on the under side and, when forced 

 down on the thin, delicate rudimentary segments, pressed them out 

 of shape, as shown in the illustrations. 



The smallest known P. transitans from York, with rudimentary 

 segments, has a length of 14 mm. to the end of the telson-like spine 



