488 FREDERICK W. SARDESON 



tion rather than retrogression. The existence of a well-developed 

 sinus in view of parallelism of development does not argue that 

 the shells are uncoiling. For the probable Lower Cambrian 

 ancestral type of Ecculyomphalus, one may as well or better 

 choose Helena (Fig. 17), a long curved cone, rather than the 

 coiled Straparollina, since it is nearer the former in stage of 

 coiling and equally far from both as to the aperture. 



The genera which are referable to Montocardia in the Calcif- 

 erous are Platyceras (Fig. 15) again and probably Holopea (Fig, 

 21) proper, the latter a symmetrical, short, spiral coil with primi- 

 tive oblique aperture, reminding one of Straparollina, but much 

 larger. With these, some species of high spiral shells, referred 

 to the genus Subulites (Fig. 16) or somewhat uncertainly to 

 Fusispira, emerge in the Calciferous. Like Holopea, they have 

 no ornamentation, but a high, closely coiled spire, shallow 

 sutures, and an elongated aperture. Between the straightness 

 of the outer side and the impress of the preceding volution the 

 sutural or posterior end of the aperture is narrowly acuminate. 

 The apertural margin diverges from the simple obliquely back- 

 ward course, or primitive aperture, by arching forward of this 

 direction from the suture across the periphery, and then back- 

 ward near the anterior end, and forward equally to the columella, 

 making a neat sinus or canal at the anterior end. 



It is unnecessary to more than mention the probable ances- 

 tral position of these Subulites shells in relation to the geologi- 

 cally later siphonate forms of the Monotocardia. The form of 

 aperture in Subulites follows the tendency, herein suggested, to 

 build forward from the obliquely and backward to the axially 

 parallel direction, the difference from Pleurotomaria shells being 

 in the position of the remaining notch, sinus, or canal, which is 

 near the columellar instead of the peripheral margin. 



In that way one can compare the Calciferous coiled shells 

 with supposed primitive dextrally coiled Lower Cambrian ones 

 with simple aperture, excepting in case of Maclurea (Plate I, 

 Figs. 10, II, 12), which is more difficult. If it is viewed as a 

 dextral coil (Fig. 12), the spire, as Billings suggested, has sunk 

 far below the median plane, in short, the umbilicus is then plane, 



