Dr. W. T. Caiman — Syncarida from Coal-measures. 493 



relative length of the sixth abdominal somite in the American specimens 

 seems to be almost the only difference available for their specific 

 separation from the English form, and it is possible that even this 

 might not survive a re-examination of the fossils. At all events it 

 seems clear that the two forms cannot be generieally separated, and 

 I propose to regard Prceanaspides as a synonym of Palceocaris. 



Comparison with recent Syncarida. — A comparison of the head-region 

 of Palceocaris as described above with that of the recent Syncarida 

 (see Fig. 2) leads to some important conclusions. There is hardly 

 room for doubt that the groove running upwards from the base of the 

 mandible on the side of the head corresponds to the so-called ' cervical 

 groove ' of Anaspides (Fig. 2, B) and Paranaspides, and the fact that it 

 dies out before reaching the dorsal surface is paralleled in Koonunya. 

 The horizontal groove behind it lias exactly the same relations as the 

 similar groove found in Anaspides and Paranaspides, though it does not 

 extend so far forward as in these genera. 



Fig. 4. Palceocaris typus. Restoration. After Packard. 



I formerly suggested 1 that the 'cervical groove ' of Anaspides, 

 which had been regarded as denning the first thoracic somite from the 

 head, should rather be identified with the ' cervical sulcus ' which, in 

 the Mysidacea, crosses the carapace immediately above the mandibles. 

 Later 2 I expressed some doubt as to this interpretation, but it is very 

 strongly supported by the evidence of the fossils now described. The 

 narrow and generally wedge-shaped segment behind the head in 

 Palceocaris must undoubtedly be regarded as the first thoracic somite, 

 and there is nothing visible in the living Syncarida which can be 

 compared with it. The term 'mandibular groove' may conveniently 

 be applied in all these cases to the groove running upwards from the 

 attachment of the mandible. 



The confirmation now afforded of previous statements as to the 

 existence of pedunculated eyes and of thoracic exopodites in the fossil 

 Syncarida goes to strengthen the case for their association with the 

 living Anaspidacea. Almost the only conspicuous structural feature 

 of the latter that has not been recognized in the former is the series of 

 lamellar gills on the thoracic legs. From the delicacy of their texture 

 it is hardly likely that these would be preserved, but it is not beyond 

 possibility that traces of them may yet be discovered. 



1 Trans. Roy. Soc. Edinb., xxxviii, p. 787, 1896. 



2 Ann. Mag. Nat. Hist. (7), xiii, p. 155, 1904. 



