951 



Sjrop*to of 



remote allinities for each other.* Beside- it leaves a large number of intermediate 

 forms which may be placed with equal propriety into either genus Hut this is not 

 all. for w believe with other author- that, as now constituted, these genera embrace 

 material more than -ulh'ciently various t admit of defining a comparatively large 

 number of vali'l generic group-, valid in the sense that they are relatively as 

 important group- ot -pecies as are ordinarily considered to be of generic value. 



Now. what are the characters upon which we propose to base these genera? 



*. upon the presence or absence and relative length of a true slit, as distinguished 

 from a mere apertural notch; second, the characters of the slit-band, among which 

 its outline, as exhibited in transverse section, is the most important; third, the 

 width of. and the position of the band and slit or notch with respect to the hight of 

 the volution*: fourth, the form of the volutions, with respect to angularity and 

 roundne--; fifth, the form of the entire shell; sixth, the character and depth of the 

 *uture; seventh, the form and outline of the aperture as shown by the lines of 

 growth, and eighth, the changes in the character of the volutions from the embry- 

 onic to the mature stages. Corroborative and subordinate characters are furnished 

 by surface markings, the apical angle, the relative size of the last volution, the 

 presence or absence of an umbilicus, and minor peculiarities of the aperture, the 

 notch, the slit and the band. The relative importance of these characters is not 

 always the same, but they are sufficiently reliable for present requirements. 



SYNOPSIS Or OBHERIC AND SUBORDINATE GROUPS OP PALEOZOIC PLKUROTOM ARIIDJt.f 



I. IX>PHOSPIRA, Whitfield, 18864 Shells with more or less elevated spires; 

 whorls closely coiled throughout or only in the upper part, the last often exhibiting 

 a tendency to become disconnected; whorls angular on the periphery and bearing 

 from one to five distinct carin.i-; central or peripheral keel strongest and most 

 prominent, carrying the band, which is obtusely rounded, or more or less distinctly 

 trilineate, with the median line heavier and more prominent than the other two; 

 rarely, if ever, solid; an umbilicus n-ually of very small size, nearly always 

 present Inner lip generally thickened, often slightly twisted, turning around 

 the umbilicus so as to form a kind of hollow pillar. Outer lip more or less deeply 

 notched, but the center of the notch, which lies at the peripheral angle, is never 

 prolonged into a -lit. Surface markings parallel with the apertural edge; occasion- 

 ally cancellated l>y linespir.il lines. Typ. M'ir. -1,1,1- mln ///>/'/!</<; Hall and J/. 

 '> 3i tor M helicUm Whitii.-ld. n..t Salt.-r.i 



To appreciate the ubordlnat*ne*> of lite ralm- of dlferrnCM In the hlbu of Uw iplre within moderate llBlU. 

 oomparteoaof U> >curM of *pMta* of t*f*wp*v deeerlbcd In thl- wurk We pe* by almoat Imperceptible 



tmdatloaa from perlr* Ilk* L. pin and L. MloMib. In wlili-h the apical angle t and decree*. to other*, like 

 L. titmtnt teford .p.. In hlrh the aaclc I. of MmeUmea ICM than decree*. And. to far w we out tee, there I* not the 

 otcct rfaaoee for a i4rte eparaUon between the Int and the la.t of the ipecta* mentioned. 



Remarkiuo (earraof whtchcpedeeandewrlbMl In t I ,1 \~ fuund oa occeedla( pace*. 



t Bull. Aner. Ma*. Nat. HIM, roLI. p. III. 



