250 



bulletin: museum of comparative zoology. 



shell is 3.5 mm. long as it lies in the matrix, but as the pygidium is 

 somewhat bent under the body, the actual length is probably .5 mm. 

 greater. The cephalon is 2 mm. long and 4 mm. wide (semicircular) 

 and is bordered by a wdde (.5 mm.) concave brim. The glabella is 

 convex, abruptly elevated above the brim at the front, bounded at the 

 sides by deep dorsal furrows which converge backward. Between the 

 eyes the glabella is marked by a pair of deep glabellar furrows. Be- 

 hind them, on the narrowest part of the glabella, is a median tubercle, 

 and on each side at this point is an isolated basal lobe. In short, the 

 glabella is like that of an adult specimen of Basilicus (Plate 1, fig. 1). 



The facial suture can not be made out with absolute certainty on 

 these smallest specimens. On a specimen with the cephalon 1.75 mm. 

 long it seems to be marginal in front, while on a finely preserved 

 cephalon 4 mm. long (No. 37), it is certainly intramarginal (Fig. 1). 



The eyes in the smallest specimens are 

 very large, and only about one half 

 their own length from the posterior 

 margin. On a cephalon 2.25 mm. 

 long, the eyes are 1 mm. long and 

 each forms a semicircle, the palpebral 

 lobes almost touching the glabella in 

 front. The posterior corner of the eye 

 is only .3 mm. from the posterior mar- 

 gin of the cephalon. The eye is there- 

 fore relatively much larger than in the 

 adult, but no further forward. The 

 growth after this stage is more rapid 

 in front of the eyes than behind them, 

 and a cephalon 3 mm. long has the eyes 

 still only 1 mm. long. 



The thorax of the smallest speci- 

 mens shows feW' peculiarities other than the narrowness of the axial 

 lobe. The specimen which is 3 mm. long has the axial lobe .5 mm. 

 wide, or .2 the total width of the thorax. 



The smallest specimen with a well-preserved pygidium is 3.5 mm. 

 long, and the pygidium is 1.25 mm. long and 2.5 mm. wide. The 

 axial lobe is very con\'ex, narrow, and is prominent all the way to the 

 end. There is a narrow concave border, and the axial lobe reaches 

 to and overhangs this border. The pleural lobes are convex, somewhat 

 triangular, and crossed by four pairs of furrowed ribs. The axial 

 lobe has one strong, and two weak rings at the anterior end. Another 



Fig. 1. Isotelvs gigas Dekay. A 

 small specimen in the Ogygites 

 stage, the facial sutures being 

 intramarginal. The pygidium 

 is incurved so that the posterior 

 part is not well shown. X 6. 

 M. C. Z. No. 37. 



