- ?-.^o - 



larger size than the type is demonstrat ant, -.Thich if 



eorh"olete, Tro^-'T'-l ''^.'^ve, a^^'iT'OTicatelj", ti- p ■''nil .-n.-iri"'' -^i'""- >i '■ i.ons : 

 lat., 55 rani., alt., 37 iau. ; semirliam., 14 mm.; incideritally the 

 hinge of this speoimen is cnriouslj- pathologic:^!, ocin,;;' inordinate- 

 ly thickened and In?-. c' arirl refler-.ed tavard W- o ••ni'hn. On the 

 larger valves the contrast hetweeii the rows of larger and snall- 

 er spines is decided, although the actual difference in size 

 is slight; but on the othe'^- ^his disparity '-'■'■• ' '-escnt, is 

 not as notioeahle. '.Then the spines are ver^^ siiall, the ventral 

 side, which is open ^■'hen the spines are lar-^e, naturally is clos- 

 ed. As us^Tal, onl, 3,rt of ^-^'^c o-'i-^amentation is intact, vrnere 

 the spines have been remove"! the ribs are smooth and flat, T^ith 

 thin margins slightly raised; if erosion proceeds further, the 

 middle of the rib is excavated smd the raised ..^^_l.„_. xenain as 

 narrow ridges. 



ihe laterally compresse'-'', triangular, ven-^-ally open spines 

 of two G-..Uxo of magnitude v.^ r,eioa "'"';-■- ■■-i '■><=•, c-- most charac- 



teristic feature of this species. Dall has descri closel- 



relate-'' form, _2» acrocoma (l), from the Ghipola; c" h smaller 



(1). rrans. V.'agner Tree Inst. oc''. ., "^^ iladelphia, vol. C , ot. 5, 

 p. 1081, pi. ^.8, fig. 2, 1903. 



it has the same outline and a siruilar type of radial ribs which 



