ROTULA AUGUSTI. 541 



than those of the lower side. The furrow in which the teeth are articulated 

 is deep ; the teeth are thin, sharp, as in Mellita. The auricles are thin, 

 broad, and flat, scarcely rising above the lower floor ; the edges form a small 

 furrow, corresponding to a sharp ridge on the lower surface of the jaws. 



D'Orbigny and Desor have proposed to separate Rotula into two genera. I 

 do not think this distinction is warranted. The marginal cuts, except the two 

 corresponding to the extremity of the lateral ambulacra, are simple marginal 

 notches, which we know, from the mode of development of similar notches 

 in Encope, to be subject to great variation, and the presence of two additional 

 closed anterior interambulacral lunules in R. Augusti is not a sufficient reason 

 for separating it generically from R. Rumphii ; there is certainly nothing in 

 the interior structure of the test warranting the separation proposed by 

 D'Orbigny, and adopted by Desor. 



Rotula Augusti 



! Rotula Augusti Klein, 1734, Nat. Disp. Ech. 



PL XIIF.f. 4-3 ; PL XXXIII. /. 5. 



The outline of the test is rounded anteriorly, somewhat truncated poste- 

 riorly, though in young specimens the general outline is almost circular. The 

 test is depressed ; vertex anterior to the apical system, which is rather eccen- 

 tric posteriorly. There has been great confusion introduced in the nomencla- 

 ture of this species, from descriptions made upon specimens of different sizes, 

 which vary greatly in the number, size, and proportions of the marginal cuts. 

 The general character of the marginal cuts is, that we have five deep median 

 interambulacral cuts extending half-way towards the centre ; the principal 

 lobes thus formed are again subdivided along the posterior edge, so as to 

 form two to four digitations in the posterior lateral and the odd interambu- 

 lacral spaces, — one or two on each side of the original median -cut ; while 

 in the anterior lateral interambulacral spaces the median cuts become nar- 

 rower with advancing size, and eventually close, so as to form lunules in the 

 interambulacral spaces. The median cuts opposite the anterior lateral am- 

 bulacra also close as lunules with advancing age. The digitations are broad, 

 rounded at the extremity, and with sides of variable size ; the two adjoining 

 the median interambulacral lino have one side much the longest, while those 

 forming the central part of the lobe have shorter sides of nearly equal 

 length. From an examination of a very young specimen we see that the 



