50 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 78 



rounded off; surface moderately convex, usually depressed in the 

 antero-dorsal region; beneath this depression and generally near the 

 lower part of the anterior edge a conspicuous though variable node or 

 ridgelike swelUng; anterior, ventral and posterior margins with a 

 flattened border, usually widest anteriorly and sometimes excavated 

 on the inner side. Test tliin, black, corneous, pliable, smooth or 

 punctate exteriorly. 



Genotypes. — Alutaflexilis Matthew (original) and Aristozoe troyensis 

 (Ford) Walcott. 



Remarks. — The above definition aims to be a composite of the char- 

 acters noted in studying the following 8 or 10 species. Together they 

 form a reasonably compact group distinguished from all others by the 

 form of the carapace and more especiall}^ by the node in the antero- 

 ventral fourth. This node varies considerably in position and devel- 

 opment, but it does not get far enough out of its normal position to 

 suggest comparison with the ocular node in Bradoria. In the latter 

 the node is strictly confined to the dorsal region of the valves; in 

 Aluta, on the contrary, it must be counted a feature of the antero- 

 ventral part. 



After cleaning the original type of Matthew's Alutaflexilis, it was 

 found that he had figured it upside down, and that the outline of the 

 specimen, instead of being regularly oval, was really obliquel}'- acumi- 

 nate oval. It has a comparatively short but straight hinge line and 

 well-defined cardinal angles. As now understood, its outline is not 

 greatly different in essential respects from A. stereope (Walcott) and 

 A. suhrotunda (Walcott). Certainly there is not sufficient difference 

 in their respective outlines to warrant generic separation. 



Unfortunately, the type of A. flexilis is a specimen so crushed that 

 it is impossible to prove that it was originally provided with a node. 

 The specimen is thought to consist of both valves, which, fitting closely 

 and consisting of a pliable substance, constituted a baglike carapace 

 that collapsed when it was subjected to the pressure of the entombing 

 sediments. The Chinese A.fragilis (Walcott) is founded on similarly 

 crushed specimens. However, the irregularities of the surface of the 

 type of A. flexilis probably are not altogether accidental, the elevation 

 between the large and two smaller depressions in the anterior part 

 of the specimen possibly corresponding to the characteristic node. 

 Believing that the specimen on which Matthew founded his genus 

 Aluta really did have such a node, and as it agrees in all other generic 

 respects with the other species here referred to the genus, it seemed a 

 better plan to redefine Aluta than to confine it to A. flexilis and erect 

 a new generic group for the other more fully known species. 



Considered individually the species of Aluta indicate rather diverse 

 alliances. Thus the smaller forms, like the Chinese A. woodi and 



