46 PALEONTOLOGY OF NEW YORK. 



of a considerable variation in size, from a length of 5 mm. to a length of 

 L20 mm., as indicated by a large fragment, and this development in indi- 

 vidual growth is accompanied, in the earlier stages, by considerable variation 

 in the form of the pygidial lobes. A pygidium measuring 1 mm. in length 

 ami 3 mm. in width to the tips of the spines, has a very elevated axis, which 

 is sub-spinose along the axial line, strongly incurved toward the extremity, 

 and hears hut six annulations. The marginal spines are unusually elongate 

 and baccillate, the first pair being longest, the length decreasing backward, 

 the two posterior pairs becoming strongly incurved toward the posterior 

 extremity. The terminal lobe is very small and scarcely visible. With 

 advancing growth the spines become relatively broader, and radiate regularly 

 from the curve of the margin, assuming a sub-equal length, and the terminal 

 lobe becomes gradually more conspicuous. Individuals occur occasionally in 

 the shales of the Hamilton group, which indicate a slight inclination from 

 the type of Dalmanites (C.) Boothi to that of the variety D. (C.) Calliteles to the 

 extent of retaining the broad, short, vertically flattened cheek-spines of D. (C.) 

 Boothi, and showing a tendency in the thoracic segments to become spinose 

 on the axis, and in the pygidial spines to become narrow and acute. Such 

 forms would naturally be expected in abundant material, but their rare 

 occurrence, makes it inadvisable to recognize the usual variation as a dis- 

 tinct specific form. 



Dimensions. A normal adult of Dalmanites (Cryphccus) Boothi affords the 

 following measurements : 



Body. Cephalon. Thorax. Pygidium. 



Length 38 mm. 12 mm. 15 mm. 11mm. 



Width 24 mm. 24 mm. 22 mm. 18 mm. 



Of Dalmanites (Cri/phmus) Boothi, var. Calliteles, the following : 



Body. Cephalon. Thorax. Pygidium. 



Length 37 mm. 13 mm. 13 mm. 11mm. 



Width 26 mm. 26 mm. 19 mm. 21mm. 



