12 Field Museum of Natural History — Geology, Vol. IV. 



than the others. Ambulacra form the greater part of the peristomal 

 margins, and are moderately broad on either side of the sternum. 

 Peristome eccentric in front, semilunar, with a well-developed pos- 

 terior labrum. Periproct at the upper part of the posterior truncation. 

 A peripetalous fasciole, and a lateral fasciole starts from the peri- 

 petalous fasciole close to the antero-lateral petals and passes beneath 

 the periproct. Tubercles crowded, increasing in size as they approach 

 the peristome, usually crenulate and perforate, and either on flat or 

 in sunken areoles. 



Range Cretaceous to Recent. 



Linthia variabilis sp. nov. Plate III, Figs. i-ii. 



Test distinctly cordate, truncated posteriorly, ventral surface 

 depressed convex, dorsal surface elevated, forming a sharp ridge in 

 the posterior interambulacral area. Sides inflated and sloping to the 

 lateral and anterior margins; posterior truncation slightly concave 

 and the angle between the truncation and the base line varying from 

 77° to 90 . Ambulacral areas straight, petaloid, situated in compara- 

 tively deep depressions of the dorsal surface. Antero-lateral pair 

 about one and one-half times the length of the postero-lateral pair, 

 poriferous zones of these four petals wide, pores elongated and slit- 

 like. Each pair of pores connected by a shallow groove and the pore 

 near the border of the petal the larger. Unpaired anterior ambulacral 

 area situated in a deep depression the continuance of which forms a 

 sulcus in the anterior margin. Poriferous zones of this area narrow 

 and situated far apart. Pores round and each pair separated by a 

 tubercle. Interambulacral areas broad and composed of large plates. 

 Surface of the test covered with minute perforated tubercles having 

 crenulated bosses. These tubercles increase in size as they approach 

 the peristome. Interspaces filled with small tubercles and microscopic 

 granulations. Both peripetalous and lateral fascioles are clearly 

 defined, peripetalous fasciole decidedly bent inward between the 

 antero-lateral and postero-lateral petals and somewhat less so between 

 the other petals. Apical system situated in the center or somewhat 

 anterior to the center of the dorsal surface; small, depressed, having 

 the four genital plates perforated and separated by five small radials. 

 Peristome transversely elliptical, situated near the anterior margin. 

 Labrum prominent. Periproct somewhat elongated vertically and 

 situated near the top of the posterior truncation. 



