182 FRESH-WATER RHIZOPODS OF NORTH AMERICA. 



somewhat regular row, along the summit posteriorly and at the sides. The 

 spines range from one to nine, but, as previously indicated, may be 

 altogether absent. They are straight or curved, awl-shaped, hollow pro- 

 cesses of the shell. In one instance, as represented in fig. 35, pi. XXXI, 

 a curved spine projected from the front of the shell. 



The shell of Centropyxis aculeata is commonl}- composed of a yellowish 

 or brown chitinoid membrane, incorporated with variable proportions of 

 quartz-sand. Often finer particles of this material are scattered over the 

 shell, while usually the largest grains accumulate in variable quantity along 

 the position of the spines on the fundus. Rarely, the shell is entirely 

 devoid of adherent or incorporated sand or other particles, as seen in the 

 specimens of figs '29-33, pi. XXXII. Often the shell appears composed of 

 sand particles, as is ordinarily the case in Difflugias, and as represented in 

 figs. 1 7-30, pi. XXXI. Sometimes the sand is substituted by thin plates, 

 probably diatom fragments, as seen in fig. 3, and sometimes by more dis- 

 tinctly marked diatoms, as seen in figs. 31-34, pi. XXXI, and fig. 34, pi. 

 XXXII. 



Especially in the Centropyxis ecornis is the shell composed of quartz- 

 sand, as seen in the figures of that variety in pi. XXX. 



The spines of Centropyxis aculeata are almost always composed of chit- 

 inoid membrane, even when the body of the shell consists of sand, but 

 sometimes these processes have particles incorporated, and not unfrequently 

 they are terminated by a sharp quartz flake. 



Mostly the shell of Centropyxis is of some shade of brown or dull 

 yellow, but may also be colorless. Not unfrequentlv the specimens com- 

 posed of sand appear to have the cementing substance stained while the 

 sand is colorless, or the former may be of a darker hue than the latter. 

 Such specimens appear as if invested by a colored net with lighter meshes. 



The construction of the mouth of Centropyxis aculeata appears to be 

 of more complex character than in most other Lobose Protoplasts, and its 

 condition seems to have escaped the notice of previous investigators. This 

 has no doubt been due to the fact that the orifice of the mouth is so deeply 

 inverted, and is commonly more or less obscured from view by the sand 

 composing or incorporated with the walls of the shell. 



In many specimens, the mouth, as seen through the shell, from beneath 

 or above, appears large, and simply circular, as represented in figs. 21, 25, 



