LITHOTRYA DORSALIS. 353 



terga ; inner growing or corium-covered surface, with its 

 basal margin, protuberant and arched. 



Rostrum (PL VIII, fig. 1 a, a, and greatly magnified 

 1 b') very narrow ; rarely more than two or three layers 

 of growth are preserved ; the sides are deeply sinuous, 

 owing to each zone widening downwards ; basal margin 

 rounded ; in width equalling about two and a half of the 

 uppermost scales of the peduncle, and about half as wide 

 as the latera. 



Later a t small, placed obliquely, and parallel to the lower 

 carina! margin of the terga ; longer axis equal to five of 

 the uppermost scales of the peduncle, and to nearly half 

 the width of the base of the carina ; growing surface (or 

 a section made parallel to the growth-layers,) is narrow, 

 elliptic, pointed at both ends, but the carinal half rather 

 thicker than the scutal half. 



The Peduncle varies in length, generally about twice as 

 long as the capitulum, in one specimen above thrice as 

 long. The upper part as wide as the capitulum, the lower 

 part sometimes much attenuated. The calcified scales in 

 the uppermost whorl (PL VIII, fig. 1 b') are only slightly 

 larger than those in the second whorl ; the scales in the 

 succeeding three or four whorls, are considerably larger 

 than those below, which latter very gradually decrease in 

 size, till, low down on the peduncle, they are barely visible 

 to the naked eye. In this lower part, they may be called 

 calcareous beads ; they stand some way apart from each 

 other ; they are nearly hemispherical, smooth, translucent, 

 and furnished with a conical fang ; some of the smallest 

 were ^th and ^th of an inch in diameter. The upper 

 scales vary somewhat in the outline, the most usual shape 

 being sub-triangular, with the lower margin arched and 

 protuberant ; and this margin, in the two or three upper 

 whorls, is crenated with teeth, which are conical and 

 sharp, after exuviation, but soon become reduced to mere 

 notches. The scales in the uppermost whorl are usually 

 nearly quadrilateral ; the imbedded portion, or fang of 

 each scale, is, in all, produced into a blunt rounded point. 



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