600 



THE UPPER SILUKIAX. 



the Trenton limestone, but a careful comparison shows it to be 

 wider and more abrupt at its posterior termination, while the 

 direction of the striae of growth is very distinctive, these marks 

 being regularly curving toward the posterior end in M. nasutus, 

 while in this species they are abruptly bent at the postero-basal 

 angle, and again on the cardinal side, corresponding with the 

 truncate posterior extremity of the shell. Arisaig, coll. J. W. D. 



Modiolopsis allied to M. subcarinatus. Arisaig, coll. J. W. D. 



Clidophorus cuneatus, Hall. Shell ovoid, gibbous in the middle 

 and on the umbones, gradually declining behind; beaks anterior, 

 sub- terminal ; anterior end broadly rounded, the posterior end 

 narrower and sub-acute, posterior umbonial slope marked by an 

 obtuse rounded ridge, which extends to the posterior extremity, 

 and below this an undefined sinus which, expanding, extends to 

 the postero-basal extremity, while a less defined ridge bounds this 

 sinuosity on its anterior side ; surface marked by fine irregular con- 

 centric striae. 



In the casts of this shell there is a strong linear straight clavicle, 

 extending from a point just anterior to the beak two-thirds across 

 the valve. Arisaig. 



Fig. 203. 

 Modiolopsis rhomboidea. 



Fig. 204. Fig. 205. 



Clidophorus conceatricus. Clidophorus erectus. 



Clidophorus concentricus, n. s. (Fig. 204). Shell sub-equilateral, 

 very broadly oval-ovate, the anterior end the broader; height nearly 

 four-fifths the greatest length ; anterior side a little shorter and more 

 broadly rounded at the extremity; a slight depressed sinus on the 

 posterior umbonial slope, which is more anterior than in the preceding 

 species. Surface marked by even band-like concentric striae ; shell 

 thin ; a linear curving clavicle extends from the cardinal line just 

 anterior to the beak more than half way to the base. 



The prominent points of distinction between this and the preceding 

 shell are the nearly central beaks, the band-like striae, and the curving 

 clavicle with the broad and nearly equal extremities of the valve. 

 Arisaig. 



Clidophorus erectus, n. s. (Fig. 205). Shell somewhat rhomboid- 

 ovate, the height and length about equal ; umbones prominent, beaks 

 nearer the anterior end, somewhat curved and elevated ; posterior 





