132 The Ottawa Naturalist. [Nov. 



King on a waterworn surface of the beach, partially covered by 

 the shingle, and many details of the structure have doubtless 

 been removed since it was first exposed to the action of the 

 elements. The outHne of the hypostoma is rather faint on the 

 specimen, but the excellent photograph made by Miss Bentley 

 brings it out clearly. The forks are rather short and far apart, 

 and the anterior portion is narrow, so that the general form of 

 the hypostoma is more like that of Isotelus harrisi than /. gigas. 

 The total length is 75 mm., the greatest width, 65 mm. ; length 

 of the body portion, 53 mm., width, 50 mm. 



The transverse furrows are the impressions left by the 

 gnathobases of the basal joints of the legs. These gnathobases 

 were evidently long and very heavy, but the specimen has been 

 so abraded that all details are obscured. The first six pairs of 

 impressions are longer and deeper than the four behind. The 

 first eight pairs seem to belong to the thoracic appendages, while 

 the last two belong to the pygidium. From the posterior tips 

 of the hypostoma to the first gnathobases of which traces are 

 present there is a distance of about 22 mm. without impressions. 

 In Isotelus gigas the hypostoma normally extends back to the 

 posterior margin of the cephalon, so that it seems that in this 

 specimen the impressions of the first two pairs of gnathobases 

 under the thorax may not have been preserved. In that case, 

 the six pairs of strong impressions may represent the last six 

 pairs of thoracic segments, and the pygidium might begin with 

 the first of the fainter ones. 



Two specimens of Isotelus, somewhat similarly preserved, 

 have been figured. One is Billings' specimen from the Trenton 

 at Ottawa, and the other was described by Mickleborough and 

 by Walcott from a specimen found near Cincinnati. Both of 

 these specimens, however, show the trilobite itself, as well as 

 the impression. Both show^ the long heavy gnathobases of the 

 coxopodites, and it becomes evident that, as the coxopodites 

 are attached directly under the dorsal furrows, the increase in 

 the width of the thoracic lobe, which is so marked a feature in 

 Isotelus, is due to the great development of these gnathobases. 

 The writer believes that this impression on the ripple-marked 

 sand of the Chazy gives a clue to the development of the gnatho- 

 bases. Apparently Isotelus was a bottom crawler, and the 

 gnathobases may have served as ambulatory appendages. In 

 both the specimens of Isotelus mentioned above as retaining the 

 appendages, the gnathobase of a thoracic appendage is nearly 

 as long as the remainder of the appendage, and being a single 

 rod, and not jointed, is much more useful as a lever. On all the 

 specimens known, the gnathobases are strongly developed on 

 the thorax, and only feebly so on the pygidium. Beecher found 



