Tkoi'idoi.eptus Fauna at Canandakiua Lakk, N. \'. Ill 



Muscle Scars. — The scars of the muscles are so faintly impressed 

 in young specimens, that they cannot be made out. In the ventral 

 valve of the adult are two large flabelliform diductors and between 

 them the narrow elongate scars of the adductors, one on each side of 

 the median septum. The scars of the muscles attached to the dorsal 

 valve are described under C. miicroiiatus. Outside the scars, in this 

 valve, the whole surface is covered by numerous radiating rows of 

 small, sharp, forward-pointing pustules. 



Spines. — The number and place of appearance of the si)ines varies 

 somewhat in the different individuals, and the bases of the earlier 

 spines become so obscured that it is often difficult to recognize them. 



On most individuals with a hinge width of 3 mm. there are 6 

 spines. On individuals in most of the neanic stages there are 6 to 14 

 spines; on adults, 14 to 20. In this reckoning, spines and spine bases 

 from which spines have been broken or worn, are counted as spines. 



The spines do not appear in exactly symmetrical positions on each 

 side of the beak and there are not always the same number on each 

 side, though usually there are. The departure from the symmetrical 

 is not great, varying from a few hundredths of a millimeter, near the 

 beak, to i or 2 mm. in case of spines near the cardinal angles. 



The first spines appear about .25 to .35 mm. from the beak, the 

 .second .54 to .75 mm. and so on. 'J'he averages of a number of 

 measurements are : For ist pair, .30 mm.; 2d, 62 mm.; 3d, i.iomm.; 

 4th, 1.62 mm.; 5th, 2.32 mm.; 6th 3. 11 mm.; 7th, 4.81 mm.; 8th, 

 6.16 mm.; 9th, 8.08 mm.; loth, 10.5 mm. It would seem that, in 

 many cases, certain of the spines were suppressed. For instance, in 

 one specimen 8 mm. wide, there are only six spines. The first pair 

 is .66 mm. from the beak, the second 1.5 mm., and the third 3.3 mm. 

 Comparing these measurements with the averages given above we find 

 that these spines are the second, fourth and sixth pairs. Very likely 

 the first pair was formerly present, but are so entirely broken away as 

 to leave no trace. In all specimens examined the second i)air could 

 be seen, and, in nearly all, the third pair. 



The spines make a small angle with the cardinal line, usually less 

 than 45°. The variation is from 18° to 43°, and the average is about 35°. 



The Same Species from Otiicr Localities. 



Specimens from Sheldrake, Cayuga Lake, N. Y., are larger than 

 the specimens just considered. An average specimen is 20 x 29 mm. 

 and has 150 stride. There is abroad shallow sinus in the ventral valve. 



