Off] 



Hjl! 



Respecting tin 1 .iliuiit to In- de-cril>ed aii<l tlm-e uhirli I have in ; 



eferrcd tn Hytlnii-i/in-i'i, it i- -r.lliciciit to >ay that in neaily CM-IT in-tancc 



they agree irith oae or another of the species which I'rof. .inm-- h.i> placed 



Under tllC geilll-. 



It may 1-e \ .ill attention tn the fact that the Silurian genii- <'>/// n'/linn, 



lones ami Hull.' i- founded upon -pecios very similar externally to some of the 

 Silurian \Vhetlier any of the latter have the obscure internal 



thirl nf the te-t which are said to characteri/.i- (''/tlien-llinn is unknown, Imt 



con-tdering the -imilarity of their external feature-, it -eems a little >t range that 

 I'rof. .lone- ha- not remarked ni,,i, it in hi- more recent writings. 



IM i HIM i-i;i- I-M.IMIKII-V 



E \I.IV F108. 



. llvi.U 1872, Twi-iily-fniir' I, pi. \ in. 



1. il, p. l"l. i r. . tig. 5. 



/.. 



Kgti. UGH, I*"'. O.iilri \, UK- 6.) 



: IOSKS, \9SO. i.in.1- . in, flga. 



iln- -1 



.1 :u)tuin : hik'lit <M >,; ne*0.5 i 



1 71 linn ; hitMit " '- mm.: tlii'-koew 0.23 in 



\- the. characters of tin- -|u'cies have been quite generally misinterpreted, I 



ihle to illustrate them as far as shown in three typical exam; 

 1 'i the t es of measurements given above, the first may \>e regarded as a fair 



ap- for fully grown specimen-, while the other i> taken from oue of the smallest 

 seen. The length usually \arie- Let ween 1.0 ami l.'J mm., ami occasionally it 



in-- \.'t mm. l-'ignri'.N 'J'.t, -\'2 ami :. though ilitiering as much in tlieir outlines 



a- any in hiui'lro<l< of val\. it little unlike each other, and thus prove, in thi- 



ny rate, the con-tancy of the The greatest variability noticed 



-lij^lit one in tlio relati --e in which the central thir-1 of the ventral slope 



is flattened or hollowed out. It is never much, yet alway- di-tin^ui-halile. The 

 valves are slightly unequal, the left, liein^' the larger, overlapping the right on both 

 the dor-al and ventral m;i 



On the inr valves i utral thickening of the t. 



noticeable. Though -light, it covers con-id. -pace, c-pccially in its \ertical 



extent, and i- of -uch a nature that it won hallow vertical furrow on casts 



HI. urn. ti, >in.|>.i|i. 



had In WS& de*crlbrd fmm cu ..f :'.. Interior a* Or yr if Ilia, mnd lb' frumlli frniinil rid The u-t :- 



ImlUrto/li*""-"-!/!.""-"'' 1 - ' r enJimirr jhrfured ' 



form of l'. UtjM'i KTvktly rMeablw B. <ltedr<aa Ball. p. 



