A. ',7I 



all. I 



grou|>. Witli these we may include, wit limit materially altering our conception of 

 the L'emis. the new I>. bilu' although in tin tin- charactei . -kle- 



ihaped marginal ridge ifl winting posteriorly. But the other Mianei . /* 



certainly a n>iiiarkalili> form. In I ireneral appearan 



well with /' and I>. tim/ifit having tin- sickle-shaped ridge well 



developed, uixl two lar^'e centro-dorsal node- ted l.y a depression, a- in tin- 



latter species. Hut the peculiar feature is that t hese nodes are prolonged lielow ami 

 united l>y a -lender connection, ^iviiiy n> precisely the horseshoe ridge of a Ro/lin. 

 The .|iie-ti(.n arising at once i-. why -hould the not \>e viewed as a Bollin, 



rather than a / 'tit. 



1 have decided for />//./>*//// on what I helieve to be good genealogical grounds. 

 In the tir-t place, a-idc from the ventral connection of the nodes, all the characters 

 of ti > tlio-e of lh,jin>i>Un. The inar^iiiiil rid^e. it is true, run> farther 



up on the anterior end than on any of the other species, yet its extremity is thin and 

 the mere fact that it i- a trille loiiL'i'r than usual cannot he of much con-f(|iience. 

 Hut the mnst im|>ortant evidence on the .|iie-tion is furni-hed hy I>. <;;///</</ var. ////- 



of which a copy of the original li^nn- 11 ahove. In tin- variety, namely, 



there is a well defined depression hetween the nodes precisely as ill D. I' and 



all that is re. in i red to produce the loop of the latter, is a slight raisin-; of the node-. 

 together with the lower border of the depression. 'This is not, 1 supposing 



too much, for a ventral coalescence of the anterior and posterior lobes or nodes is 

 not liyanymea to /,'///,;. I mleed it occurred under one form or another, 



among many types ( ,f li,i/,-ir/i //./,/. That this is true, a glance at plate XLIV may 

 siiflice to prove. One form is shown in ti^. 4. another, and widely different, in i 

 while s. in. 1-J. ]:,. 17. j ... and -_>7 illustrate other types O f the same condition. 



DISK i- AN KM. A mi. \TKI:AI.I~. //. 



I'l.ATK XI.VI 



/lit I '.4 nun : Kmlest 1 



--. ill. .ml I .limn.; tliirkti--- .lit li.li mm. 



Valves sul,,,val or ol)lon^-siil..|uadrate. compressed; dorsal margin 

 distinctly aiiu'iilar at the extremit ; a trifle narrower, and the outline 



less convex than the posterior: ventral margin m iit centrally. Kiiiinin^ 



parallel with and close to the ventral margin a stron. at thickened at 



i end, hut not continuing up the ; 



this two irregularly triangular and ver\ prominent lar^e no-le- extend to the dorsal 

 edge. U'von-l which their pointed extrem nally jiroject. The la-t i- true 



aUo of a small central tul.ercle. 



