52 CEYLON PEARL OYSTER REPORT. 



whole uterus in the ripe proglottides is thus somewhat dumb-bell-shaped, the narrow 

 part being pinched in by the cirrus bulb. The ova are slightly oval. 



At first it might be thought that we had to do with a tapeworm of the same 

 genus as LINTON'S Lecanicephalum, but on referring to his description* and figures it is 

 evident that this can hardly be the case. LINTON describes the head as " consisting 

 of two disciform plates," but he figures a round anterior part lying in the foremost 

 plate. The figure is very poor and we may be wrong in this interpretation, which, 

 however, is strengthened by his comparison of Lecanicephalum with the Discobothrium 

 of VAN BENEDEN, for in this genus the head is divided into three distinct parts. 

 The disks of Lecanicephalum are nearly circular, and the posterior bears four suckers, 

 but there is no mention of any papillae projecting from them. VAN BENEDEN'S genus 

 Discobothrium found in Trygon pastinaca is figured, but not described by him.f It 

 has two distinct circular disks, corresponding with the rounded anterior part and the 

 first disk of Lecanicephalum, and then a thicker somewhat cruciform disc with very 

 salient angles which terminate in suckers. Our specimen has the anterior round 

 part the curling stone as we have described it resting on a quadrangular cushion 

 with suckers at the angles. On the whole it seems that we must describe it as a 

 new genus, and this we do under the name Tetragonocephaluvn, from the square 

 cushion which forms the larger part of the head. 



Tetragonocephalum aetiobatidis, n. sp. Plate, figs. 9 and 10. 



A single specimen of another Cestode of apparently the same genus was found 

 with those described above in ^tiobatis narinari. Its length was 1'3 ceutims., and 

 its breath, which was remarkably uniform behind the head, was 0'5 millim. (Plate, 

 fig. 9). The head was three times this breadth and consisted of a rostellum, long 

 and conspicuous and unarmed, and of a swollen base, squarish in cross-section, with 

 four small suckers at the anterior angles (Plate, fig. 10). Posteriorly the basal 

 portion overlapped the anterior proglottides. There is 110 neck, but the proglottides 

 appear immediately after the head, at first very narrow but with marked constrictions ; 

 as they increase in size the posterior angle becomes salient, less so, perhaps, than in 

 Staurobothrium cetiobatidis, but more so than in Tetragonocephalum trygonis. The 

 last three proglottides are twice the length of those which immediately precede them 

 and this growth is somewhat sudden. 



The head, though it differs greatly in its proportions, resembles in essentials the 

 head of T. trygonis. The marked saliency of the posterior edge of the proglottides 

 separates off the species in question from the species which inhabits Trygon ivalga. 

 As there was but a single specimen, it did not seem advisable to cut it, and as it was 

 preserved in osmic it was not possible to make out anything of the internal anatomy. 



The definition of this genus is as follows : 



* ' U.S.A. Commission of Fish and Fisheries,' 1891, p. 802. 

 t ' Mem. Ac. Belgique,' xxxviii., 1871. 



