OPODA 



Inlr. - 



dicular t.. the Mirface :iinl mostly at right angle- to the externul lines of grow t h 

 I lie lamell.e of tin- inner layer, which i~ commonly tlu> st congest of the three, are 

 also perpendicular Imt their direction is opposite. Tertiary Gastropoda often retain 

 the miciox-opic structure of the shell ina very satisfactory manner, but as a rule the 

 minute details arc oli-curc. when not entirely old iterated, in fossil species. 



The inn of the shell varies greatly, yet within the limits of genera ami -pecie- 



remarkably constant. Three types are distinguished, the tnlmliir, of straight 

 or only -lightly curved form and occurring only among the .sV,/yi/i(iy;./'/ and /' 

 ln; the .-.-//HI// in which the shell is either conical or patelliform (An-liinn- 



or involute i ll<llrr<>jilion); and the spiral. 



Tli- y far the most common type, and may in fact be looked upon as the 



typical form of the shell in tin >poda. In it the shell is essentially a spirally 



wound, elongated, conical tube, the coils or " whorls " being in most cases in contact 

 and tightly cemented or amalgamated where they join. Sometimes the whorls 

 are coiled nearly in the same plane, when the shell is said to he "discoidal." as 

 in many of the i'.nninfiluilili . More generally, however, the whorls are wound 

 al'i.ut the axis in an oblique manner, a true spiral being formed, the shell becoming 

 "turn-ted." "trochoid." "turhinated," etc. Occasionally the last whorl rises above 

 the first or apical portion of the spire (Ophileta, Maclurea), but usually the embry- 

 onic shell or "nucleus" is at the top of a cone formed by the gradually enlarging 

 and descending whorls, the mouth or "aperture" occurring at the extremity of the 

 last ami largest whorl, termed the "body-whorl." The whorls above the last 

 constitute the "spire" of the shell. The line or groove marking the junction of the 

 whorls is called the "suture." A shell is said to be "im perforate" when the axis 



columella" is solid (Fusispiru), or it is "perforated" when the axis is hollow 

 (Trochonetna), the axial cavity itself being known as the "umbilicus." The "peris- 

 tome" refers to the margin of the aperture; it is composed of an outer and an inner 

 olutnellar) Up. of which the former is often expanded (Bucanospira, Salpingos- 



) or fringed with spines. The peristome may be continuous or "entire" or it 

 may l>e interrupted or "incomplete," in the latter case the left side of the aperture 

 lii-ing formed only liy the l.ody-whorl. Not infrequently the aperture is drawn out 

 and notched below, or there may be two notches, the second being above near the 

 suture. These serve to protect the respiratory siphons. The posterior (upper) notch 

 i- prolialdy represented by the median slit and perforation in .SV/.V.SH/V//./ and / 

 relln, and the same perhaps is true of the slit or notch in the outer li; ' ///IM 



and I'lfurotomarin. 



In mo-t spiral shells the whorls normally are wound to the right the aperture 

 when in \iew heingon the right hand. In others, as for instance the recent 



