4 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 79 



CRUSTACEA 

 MARRIA WALCOTTI, new genus and species 



Plate 4, Figures 2, 3 ; Plate 5 



Two specimens (from the famous fossil bed on Mount Stephen, 

 Loc. 14s), when seen with the naked eye are amazingly suggestive 

 of a graptolite such as Nemagrapfus gracilis. They were laid aside 

 bj^ Doctor Walcott with the other supposedly Cambrian graptolites. 

 After the study of these two specimens on which the following 

 description and discussion are based, further search yielded five 

 more incomplete ones, most of which had been regarded as frag- 

 ments of the sponge Pirania muHcata Walcott.^ 



When the specimen selected as the holotype was studied under 

 the microscope it lost its graptolitic aspect and revealed itself as 

 the segmented body of a crustacean with large regularly jointed 

 arms, each joint of which gives rise to a side branch. In other 

 words, it is a bizarre crustacean, its immense swimming feet serving 

 to distinguish it from all other Cambrian crustacean genera. 



Inasmuch as Marrella may become a synonym, if my subsequent 

 contentions are sustained, and thus nullify the compliment that 

 Doctor Walcott wished to pay his friend Prof. John E. Marr, of 

 St. Johns College, Cambridge University, I am calling this new 

 crustacean Marna in order to perpetuate the compliment. 



Des&nption. — Body small (7.5 mm. long and 3.5 mm. wide in com- 

 pressed condition), elliptical in outline, with truncated front. Cara- 

 pace of head (or cephalothorax?) of subquadrangular outline (about 

 3.25 mm. long and 3.5 mm. wide) occupying half of the body. 

 Postcephalic portion (either thorax + abdomen or abdomen only) 

 consisting of seven (or possibly eight) simple segments, the first of 

 which is 0.7 mm. long, the others decreasing slightly in length as 

 well as regularly in width. There is no trace of a telson or of caudal 

 styles. The frontal portion of the supposed head possesses a sub- 

 triangular depression, the base of which is in front. Near the apex 

 is a small tubercle with a central depression, strongly suggesting the 

 presence of an eye. Since the surfaces of the head and segments show 

 no sculpture, they were apparently smooth. On the head, to the left 

 and right and behind the eye, are several irregular nodes, which may 

 be incidental to the preservation. There is also a pair of black spots 

 or minute tubercles on either side of the eye. A distinct tubular de- 

 pression, suggesting the alimentary canal, begins behind the eye, 

 where it is somewhat wider, and extends backward to the first 

 segment. 



The most important feature of this organism is the presence of 

 the two pairs of immense swimming appendages, both of wliich 



1 Walcott, 0. D., Smithsonian M'isc. Coll., vol. 67, p. 298, pi. 79, fig. 1, 1920. 



