Weno and Pawpaw Formations 81 



suiting cross-section of the last turn is thickly crescentic (pi. 2, fig. 1). 



The remaining individuals have thick coils but show a less rapid ex- 

 pansion of the last volution. One individual (pi. 2, fig. 7) has preserved 

 most of the uncoiled portion which is a thick crescent with rounded sides 

 and a relatively narrow non-ornamented dorsal groove; the aperture is 

 destroyed. Usually only the coiled portion is preserved, and these in- 

 dividuals are of variable size, even when the dorso-lateral tubercle is also 

 present. They have the same relative thickness and expansion of the coil 

 as the individuals of S. aequalis figured by Pervinquiere. 1 It is notable 

 that the ribbing becomes obsolete over the lower half of the dorso-lateral 

 tubercle and the entire apertural region. 



A straight groove of variable prominence along the ventral midline is 

 a feature of all material found. 



RIBS : The ribs are branched or unbranched. They are most numer- 

 ous at the ventral mid-line and thence laterally some stop short of the 

 dorso-lateral margin and some pass over this margin with a pronounced 

 sigmoidal flexure and describe across the dorsum an arcuate curve, con- 

 vexed towards the aperture. These principal ribs are thick and elevated 

 laterally. In addition there are shorter ribs which are best developed 

 at the venter and disappear at both ends, or else branch from a principal 

 rib laterally, and crossing the venter disappear laterally, on the opposite 

 side of the coil. There is some irregularity in the distribution of these 

 types of ribs, as follows: In one invividual (pi. 2, fig. 7) in the region 

 at the end of the uncoiled portion there are long and short simple ribs 

 and branched ribs but the long simple ribs predominate. In the branched 

 ribs the point of branching is far ventral, lying one-third the distance from 

 the ventral midline to the dorsal midline. Near the dorso-lateral tubercle 

 there are long and short simple ribs with rare incipient branching; the 

 short ribs decrease in length, as their ends lie farther from the umbilicus. 

 The same statements hold for the other individuals figured. In the distal 

 half of the outer volution of the coiled portion a short simple rib alter- 

 nates with a -long simple rib ; however, there is incipient branching in the 

 long ribs opposite the dorso-lateral tubercle (pi. 2, fig. 7) sporadically 

 near the tubercle (pi. 2, fig. 4) or even over the whole distal half of the 

 volution (pi. 2, fig. 12). In one individual (pi. 2, fig. 8) there are alter- 

 nate long and short simple ribs next to the tubercle, but more proximally, 

 there is an irregular alternation of simple and branched ribs. In general, 

 this region shows an alternation of long simple ribs with short simple, 

 ones, and incipient or developed branching. This is the situation in the 



'Et. Pal. Tun., plate IV, figures 24-26. 



