CI.ARK.] CASSIDULUS &UBCONICUS. 71 



The mouth opening is small, pentagonal, and surrounded by a well- 

 developed lloscello. The anal opening is round and situated high 

 above the margin, in a deep sulcus, that becomes narrower and shal- 

 lower toward the ambitus. 



Related forms. — Tliis species is most closely related to Cassidulus 

 subco7iicus,hnt the latter is much more elevated, while the anal opening 

 is situated higher and in a less depressed sulcus than in C. fmbquadratus. 



Locality and geological horizon. — This form is IVom the lli|)h\v forma- 

 tion (upper Cretaceous) of Mississippi. Tlie specimen dcscri I )l'(1 is from 

 Holly Springs, and was collected by Mr. W. J. McGee. 



Collection. — U. S. National Museum. 



Cassidulus subconicus Clark. 

 Plate XXXII, Figs, la-lc, 



Cassidulus snhconicus Clark, 1891. Johns Hopkins University Circnlars, No. 87, p. 76. 



Determinative characters. — Test subconical; margin subpeutagonal, 

 truncated and rostrated posteriorly; apex anteriorly jilaced. Ambu- 

 lacra broad in petaloidal portion, narrow beyond; poriferous zones 

 unigeminal, pores witli slit-like openings in outer row of x'etaloidal 

 areas. Axjical disk large, anteriorly situated. Mouth opening subcir- 

 cular, with very prominent floscelle. Anal opening large, situated high 

 on upi)er surface in shallow sulcus. 



Dimensions. — Length, flinches; width. If inches; height, 1^ inches. 



Description. — The outline of this form is so similar to the one preced- 

 ing that there was some hesitation, at tirst, in establishing it as an inde- 

 pendent species. More careful examination of numerous specimens of 

 each species has proved the constant occurrence of the individual differ- 

 ences. Cassidulus subconicus is, as its name signifies, subconical. In 

 marginal outline it is .subpentagonal, the posterior border more or less 

 rostrated and clearly truncated. The apex is slightly forward of the 

 center, giving a somewhat sharper slope to the anterior portion of the 

 test than to the posterior. 



The ambulacral areas are broad in the petaloidal portion, beyond 

 which they are narrowed. At the margin they again acquire consid- 

 erable width, but gradually contract beyond, altliough expanded for a 

 short distance in the phyUoidal area near the mouth opening (PI. xxxii. 

 Fig. le). At the i)eristome the final jniir of plates is highly attenuated 

 (PI. XXXII, Fig. !</). The pores of the outer rows, in both the petaloidal 

 and phylloidal areas, are in a marked degree elongated. In the inter- 

 mediate portion of the column they are small and indistinct. 



The iuterambulacral areas are more prominent than the ambulacral, 

 which are slightly depressed. The snrface is covered with minute 

 tubercles and a microscopic granulation (PI, xxxii. Fig. 11). 



The apical disk is large, and composed of fonr perforated genital 



