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



II. Further Note on" Pleurocaris annulatus. 



Since the publication of ray paper on this species ' I have been able 

 to examine a very successful impression taken from one of the British 

 Museum paratypes (I 13814) by Mr. T. H. Withers, and showing some 

 features that were overlooked in the examination of the fossil itself. 

 The most important of these relate to the upper surface of the head 

 (Fig. 5), which was not displayed in any of the other specimens 

 examined. 



The anterior margin of the head is produced into a short, bluntly 

 pointed rostrum. On each side of this is an impression that most 

 probably represents the basal segment of the antennule, although 

 it is not impossible that it may be an eye-stalk. The head-region is 

 divided by a strongly-marked transverse groove into two portions, of 

 which the posterior is wider than the anterior, and more than half its 

 length measured to the tip of the rostrum. On each side of the 

 posterior division is a swollen area marked off by a groove, which has 

 an oblique and sinuous course from the antero-lateral corner to the 

 posterior margin, which it reaches at a little distance from the 

 middle line. 



Fig. 5. Pleurocaris annulatus. Restoration of dorsal surface of head-region, 

 based on an impression taken from one of the paratypes (I. 13814) in the 

 British Museum collection. 



Behind the head-region just described this specimen shows very 

 clearly that there are only seven free thoracic somites, all of them 

 provided with expanded pleural plates. It is therefore possible that 

 the posterior division of the head-region, marked off by the transverse 

 groove, may represent the first thoracic somite. On the other hand, 

 the groove seems to lie just about the region of the mandibles, and 

 well in front of the position where the anterior limit of the first 

 thoracic somite might be expected. It seems much more probable, 

 therefore, that it is to be identified with the ' mandibular groove ' 

 described above in Palceocaris. If this be so the groove defining the 

 swollen area on each side may very well represent the ' horizontal 

 groove ; of Palceocaris and Anaspides. 



Apart from the morphological interpretation of the parts, however, 

 the dorsal aspect of the head-region presents a very close similarity to 

 that described and figured by Packard in Acanthotelson? In both 

 cases the head-region is divided by a transverse groove into two parts, 

 of which the posterior is the longer and the anterior is provided with 

 a short rostrum ; in both cases also the posterior division has on each 



1 Geol. Mag., Dec. V, Vol. VIII, p. 156, 1911. 



2 Mem. Nat. Acad. Sci. Washington, iii (2) ; Mem. 15, p. 124, pi. i, fig. la, 1886. 



