308 SMITHSONIAN MISCEIJ.ANEOUS COLLECTIONS VOL. 53 



the Mesonacidcie. At the next stage recognized, fig. 6, there are ten 

 thoracic segments, a Holmia pygidium, and an enlarged third thoracic 

 segment that is typical of Mcsonacis and OlcncUus. In fig. i, pi. 33, 

 the fifteenth segment is almost a typical telson of Olenellus thomp- 

 soni, but there are three short rudimentary segments and a pygidium. 

 With another slight change the segments -and pygidium would dis- 

 appear and a true Olenellus thompsoni, like that of pi. 34, fig. 9 ; 

 pi. 35, fig. I, would result. 



The conclusion from the foregoing is that the thorax of Pcvdnmiias 

 passes through several stages of development of which we now 

 have some information. These are : 



First. Holmia stage.— A Holmia without large third segment or 

 telson. 



Second. Intermediate stage. — A form with large third segment, 

 but without a dorsal spine on the fifteenth segment. 



Third. Pcrdeuinias. — A form with large third segment, large 

 spine on fifteenth segment, and with segments and plate-like py- 

 gidium posterior to the fifteenth segment. 



Nearly all the specimens of Pcedeumias 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 Olenellus thompsoni 

 [pi. 35] and Mesonacis vermontana, from Vermont [pi. 26, fig. i] 

 the anterior lobe touches the frontal rim. With this in view, all of 

 the specimens with the rudimentary segments and pygidium from 

 Vermont and York may be considered as the Pcedeumias stage of 

 development of the Mesonacidge. The Pcedeumias segments of the 

 York specimens are short and without defined pleural lobes [pi. 33. 

 figs. 2 to 5], and in this respect are similar to those of the Vermont 

 specimen represented by fig. i, pi. 33, and fig. 12, pi. 24. 



Azotes on the young cephalon from Alabama. — Specimens from the 

 vicinity of York are all more or less compressed and flattened in the 

 shales. A fortunate find of uncompressed specimens of the cephalon 

 of some of the younger stages of growth associated with the adult 

 cephalon in the Montevallo calcareo-argillaceous shales near Monte- 

 vallo, Alabama, made by Mr. Charles Butts and Mr. T. E. Williard 

 in 1906, show some, interesting characters of the species not shown 

 by the York specimens. These are illustrated by figs. 18-22, pi. 25, 

 and may be compared directly with the young cephalon of Ellipto- 

 cephala asaphoides on the same plate : fig. 22 with figs. 9 and 10 ; 



