67-4 ON DIPELTIS AND PROTOCARIS—SCHVCHERT. vol.xix. 



gins somewhat thickened; just within the anterior margin, two large 

 nearly circular dei)ressions appear to be present, which may represent 

 eyes. Abdomen tapering slightly, with tliirty short, eciually broad 

 segments back of the carapace, and a wide anal segment. Cercopoda 

 not long and comparatively thick. 



Type.— P. marshii, Walcott. (No. 15400 U.S.^^.M.) 



The writer is unable to add much to the structure of Profocaris as 

 worked out by Mr, Walcott. The obverse and reverse sides of the only 

 specimen show that the abdomen continued beneath the cephalon, as in 

 Ajms. Therefore many more than thirty segments were present in 

 P. inarshii. The cercopoda are shorter and thicker than in Apxs or 

 Lepklurus. That eyes are present is not certain, but faint markings 

 exist as indicated in Fig. 1. Theoretically it is probable that eyes are 

 present on the dorsal surface and near the anterior margin of Protoca- 

 ris.' The reasons for this are: (1) In the development of trilobites the 

 eyes are known to pass from the ventral to the dorsal side; (li) the 

 owtog&ny of Apus shows a similar transposition; (3) in the Lower Cam- 

 brian nearly all trilobites associated with Proioairis have eyes on the 

 dorsal surface; (4) Protocaris is clearlj^ one of the Apodidic, and in this 

 family the eyes are never much removed from the anterior margin. 

 Therefore it is not to be expected that at the very base of the Cambrian 

 the eyes of Protocaris would have traveled much back of the anterior 

 margin; (5) in Bipclih of the Carboniferous the eyes are situated as in 

 recent species of the Apodidie, 



Protocaris is a synthetic type, as might be expected of animals occur- 

 ring at the base of the known fossil-bearing strata. Kegarding this 

 fossil, Walcott stated that "this is probably the oldest Phyllopod crus- 

 tacean known, and that this animal is Apus-like." The numerously 

 segmented abdomen, the i^resence of cercopoda and a wide anal seg- 

 ment, or telson, and the posterior extension of the carapace over the 

 abdomen clearly indicate that Protocaris is nearly related to the Apod- 

 idu'. On the other hand, the subquadrangular shield of this genus is 

 quite unlike that in any member of the Apodidse, and its anterior exca- 

 vated margin recalls certain Ceratiocarida^,as Ceratiocaris, Ditht/rocaris, 

 and Ar</a.s. Protocaris ditlers, however, from these Pliyllocarida in its 

 Apus-like abdomen. 



Since writing the above it has been learned that Clarke and Beiuard 

 have reached similar conclusions. The former states: 



Another very early univulved speties, not uulike Nehalht, but wouderfnlly similar 

 to the living Phyllo]»od ./jj«s, is the /Vo/oc«ri8 w«?«/u/, Walcott. * * * The single 

 example of Protocarin known, has i)robably been subjected to some horizontal dis- 

 tortion in the shale, giving the carapace a disproportionate size with reference to 

 Jpiis, ])ossibly also serving to obliterate any external evidence of ocular nodes whidi 

 may have existed, but the remarkable closeness iu the form of the abdominal seg- 

 ments, the degree of segmentation, and the single strong pair of caudal processes, 

 render it highly probable that iu Protocaris, we have to do with an apudiform phyl- 

 lopod rather than with a nebalioid phyllocarid.' 



'Ou the structure of the carapace in the Devonian crustacean Phniocaris, by J. M. 

 Clarke, Amer. Nat., 1893, p. 799. 



