SONNINIA TRIDACTYLA. 393 



similar appearance which have very different septa ; and with one of these (see 

 PL XCIV, figs. 1, 2) the present form has been placed on the same plate for 

 comparison. Side view reduced to two-thirds natural size is shown in PI. XCIV, 

 fig. 3 ; septal margin, fig. 4 ; outline of front view, fig. 2 of PI. XCV. 



/3. Costae weak. 



Sonninia tbidactyla, 1 8. Buckman. Plate CI, figs. 1 — 3. 



Discoidal, compressed, hollow-carinate. Whorls ornamented with alternate 

 costse and spinicostae, later with not prominent, nearly direct, ventrally-inclined, 

 alternate large and small costae, — the small, which gradually become obsolete, 

 representing the previous costae ; the larger, the previous spinicostae. Ventral 

 area arched, divided by a small, fairly-defined hollow carina. Inner margin 

 fairly defined, nearly upright. Inclusion about two-fifths, up to the spines. 

 Umbilicus concentric, graduate, ornamented with a regular spinous stage up to 

 24 mm. diameter (diameter of shell about 35 mm.), later with alternate costae and 

 spinicostae. Whorl-section gibbous-sided, oblong. Septa with somewhat short 

 lobes, L symmetrical, its terminal lobule axial, isosceloid, equicellate, its lateral 

 lobules equipoised, nearly isometric. 



This specimen is presumably immature. The ornamentation of the inner 

 whorls, the septa, and the compression suggest relationship with Sonn. spinosa ; 

 and the two former characters also have resemblance to those seen in Sonn. 

 arassibullata. The wider stem of the superior-lateral lobe and the shorter 

 terminal lobule seem to be against direct genetic connection with Sonn. spinosa. 

 The paucity of costation rather favours connection with Sonn. crassibullata, but 

 the septation negatives this idea. 



If all its details be taken in consideration, it is difficult to confound this 

 species with any of those figured. From Sonn. spinifera, which it somewhat 

 resembles, the longer spinous stages distinguish it externally, while the symmetrical 

 L, with axial terminal lobule, is a decided internal difference. From Sonn. crassi- 

 nuda a larger umbilicus and more pronounced ornament separate it ; from Sonn. 

 laevigata greater spinosity . and more pronounced ornamentation, as well as the 

 symmetry of the superior lateral lobe, are features of distinction. 



In septation this form agrees with Sonn. dominatrix. It is, however, more 

 umbilicate, and more compressed ventrally ; it has much longer and more marked 



1 TpidaKrvXos, three-fingered, in reference to the superior lateral lobe. 



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