GASTROPODA. 



Marlurva blj.byl ) 



MAI-I.I-KEA BIO-HYI Hull. 



PLATE I. \ \ v I 108. *-!. 



V iurra bi!/n>>yi HALL, 1861, OeoL Rep. WIs., p. 37; WHITFIELD, 1882, Geol. of WIs., vol IT, p. 222, 

 pi. vi, flgs. 17 and 18; also 1896, Mem. Am. Mus. Nat. Hist., vol. I, p. 62, 

 MI, flg*. 14 and 15 (not 12 and 13). 



i.f mi-ilium *./. ranging In diameter from 25 to 80 mm., usually 60 to flO mm. ; normal night 

 varying from flve-tw. -Ifth- t<> one-half of the width. Lower surface flat, the outer angle subacute; In 

 casts more or leas obtuse, and with the Inner whorls somewhat rounded and the suture* generally much 

 more distinct than <>n the shell Itself. Umbilicus deep, rather abrupt, exposing half or more of each of 

 the mi., r whorls. Its width always greater than one-third of, and sometimes exceeding half the width 

 <>r Hi-' entire -hell ; margin of umbilicus angular In shells, abruptly rounded in casts, the slopes convex on 

 each whorl. CasU rarely retain traces of the surface markings. These consist of more or less obscure and 

 Irregular transverse lines and undulations, crossed on the peripheral region only by revolving lines a mm . 

 or less apart. Operculum much as In M. logani, except that the nucleus Is at the lower Inner angle Instead 

 of In the center of the lower side. 



The original localities for this species afford two closely allied species, a small one that Is about 25 

 mm. in diameter, and a larger form that commonly is more than twice as wide. These two forms differ 

 furtli.T In the relative size of the umbilicus, Its width In the smaller form being in no observed case more 

 t M.in ..iK-third of the width ..f tin- shell, while In the larger form it generally equals one-half. Under the 

 rln-iinistances It Is highly probable that both forms were Included by Hall In his M. bigtbyi, and It Is a 

 mat NT of considerable difficulty to decide as to which of the two Is the better entitled to retain hi- name. 

 Tin- larger species being the more common and widely distributed, and undoubtedly the same as the best 

 preserved and largest of the original types of the species figured by Whltfleld in 1896, we have decided to 

 restrict the application of the specific name bigtbyi to It, and tx> propose the new name nitula for the other. 



formation and totality. Stones River group, Belolt, Janesvllle, Mineral Point, and other localities 

 In southern Wisconsin; Dlxon and La Salle, In Illinois; Lebanon and near Knoxvllle, Tennessee. Dr. 

 F. W. Sardeson catalogues the species as occurring In the Vanuxemla bed In Minnesota, but we have not 

 been able to verify Its occurrence In the state. 



. Geological and Natural History Survey of Minnesota; E. O. Ulrlch. 

 Register, No*. 7306, 7349. 



BIOSBYI var. DIXONENSIS n. van, and MACLURKA KNOXVILLENSIS 

 n. sp. (Vlrich.) 



(Not figured.) 



Tw<> other species of this type, both three Inches or more In dlameu-r, arc known to us, In the one 

 from rocks holding M. bigibyi. In the other In strata supposed to be equivalent or nearly so. The first, 

 which was collected at Dlxon, Illinois, Is more depressed and has the margin of the umbilicus, which 

 takes up about one-third of the diameter (if the shell, moved farther Inward so that Its walls are almost 

 v.-rt iral. As the form is easily recognized, we suggest the provisional designation M. bigtbyi var. (ttzmtensu. 

 The other was received from Prof. J. M. Safford, who collected the shell and opercula In the vicinity of 

 Knnxville. Tennessee. As It deserves to rank as a distinct species, we propose the name Mnrlunn knx- 

 nUnuu. With a general aspect like Jf. Ujpfcyt, It differs decidedly In having leas angular whorls and deep 

 sulurea on the flat side. The operculum, so far as the position of the nucleus is concerned, Is more like 

 that of Jf. logani than Jf. bigtbyi, but <l ngly from both In the fact that the nucleus is extremely 



prominent and twisted, recalllng,"somewhat feebly, a small ram's horn. 



