ART. 27 NEW MIDDLE CAMBRIAN FOSSILS RUEDEMANN H 



together, are the cast-off tests of molting individuals, had remarked : 

 " It is possible that, among others, the soft-shelled crustaceans 

 described by Walcott from the North American Middle Cambrian as 

 Man^ella and Molaria, whose systematic position could not be estab- 

 lished thus far, may be soft-shelled crustaceans, namely, trilobites 

 immediately after molting" (translation). 



Working with this suggestion in mind, the writer found from 

 Walcott's figures and the material in the New York State Museum 

 the following evidence: 



(1) The tests were so delicate that the animals clearly had insuffi- 

 cient protection. Walcott (1912, p. 194) states that in camp the 

 fossils were called the " lace crab " because of their delicate tissue. 

 It is probable that these young freshly molted individuals, in seek- 

 ing the protection of deeper and darker places, as molted crustaceans 

 do, had the misfortune of sinking into the trap formed by the water 

 charged with carbonic acid that filled the particular depression in 

 which the Burgess shale with its amazing number of species was 

 deposited, just as the other amazing accumulation of organisms was 

 formed. 



(2) Walcott saw the distinguishing characters of his family Mar- 

 rellidae in the " small subquadrangular carapace," " the two postero- 

 lateral spines comparable with the lateral lobes of the carapace of 

 Apodidae," and " the five pairs of appendages of the head." 



It is these appendages that furnish the solution of the problem. 

 Though the antennae are clearly the same as those described in the 

 trilobites, the strangest appendages and those most divergent from 

 trilobites are the two pairs of long, thick horns. Walcott termed 

 the first of these " antennulae (?)" and the other "the posterior 

 spines or lobes of the carapace." 



In most specimens the first pair projects horizontally, or sideways, 

 coinciding exactly with the frontal margin of the trilobite cephalon, 

 including the genal spines. The other pair is most frequently turned 

 so far backward that it flanks the two sides of the trunk. There are, 

 however, many specimens, and among them some that have suffered 

 little disturbance in preservation, that have the " posterior " spines 

 projecting sideways, as in Walcott's Plate 26, Figures 3 and 5. In 

 this position they correspond exactly in location and outline with 

 the posterior margin of the cephalon. 



These two pairs of appendages may, therefore, very well have been 

 the strands of thicker connective tissue, supplied with blood vessels 

 and nerves that lay under the frontal doublure and the posterior fur- 

 rowed margin of the cephalon and between which the thin membrane 

 of the cheeks was stretched out. This assumption is well supported 

 by the fact that the posterior margins of the anterior appendages and 

 the anterior (outer) margins of the posterior appendages distinctly 



