COMPOSITA 493 



Remarks. This species has been established upon two very perfect 

 internal casts from chert, which differ so notably from other allied 

 athyroid shells in form and general proportions that one is forced to 

 consider them as specifically distinct. They resemble, in some degree, 

 some of the shells referred to Athyris ambigua by British paltontologists, 

 but the interpretation of this species has commonly been so broad as to 

 include almost all species of Composite. In these internal casts the 

 external surface characters are, of course, lacking, but the shells must 

 have possessed very strong concentric lines of growth towards the front 

 because they are so well exhibited even upon the internal casts; whether 

 or no they also possessed other markings is not possible to determine 

 from these specimens. When preserved with the shell present, the species 

 could undoubtedly be recognized from its general form and proportions. 



Horizon. Residual chert, probably of Keokuk limestone. 



COMPOSITA GLOBOSA n. sp. 

 Plate LXXXII, Figs. 21-26 



Description. Shell small, subovate in outline, narrowly truncate or 

 slightly emarginate in front, usually a little longer than wide, but some- 

 times wider than long, the greatest width near the mid-length of the 

 valve ; the valves subequally convex. The dimensions of a nearly perfect 

 specimen are : length of pedicle valve 12.5 mm., length of brachial valve 

 11.1 mm., greatest width 12.1 mm., thickness 8.3 mm. 



Pedicle valve with its greatest depth posterior to the middle, the sur- 

 face curving abruptly to the cardinal margin, and more gently to the 

 lateral and anterior margins ; the beak pointed and incurved ; mesial sinus 

 obsolete towards the beak, originating as a flattening of the valve in 

 the umbonal region and continued to the front margin as a merely flat- 

 tened band or as a very shallow sinus not very sharply defined from 

 the lateral slopes. 



Brachial valve with its greatest depth posterior to the mid- 

 dle, the surface curving rather abruptly to the cardinal margin 

 and more gently to the anterior and lateral margins, not compressed 

 towards the cardinal extremities; the median portion of the valve not 

 differentiated from the lateral slopes posteriorly, but anteriorly it becomes 

 flattened or slightly depressed in a shallow sinus, which, meeting the 

 similar flattened band or sinus of the opposite valve, gives rise to the 

 anterior truncation or emargination of the shell. 



Surface of both valves nearly smooth, marked only by faint lines of 

 growth which are sometimes crowded and more conspicuous near the 

 outer margin of the shell. 



Remarks. This is a small species, rarely attaining a greater length 

 than 15 mm. It is characterized by its more or less globose form, nearly 



