40 Prof. M'lntosh's Notes from the 



VI. — Notes from the St. Andrews Marine Laboratory {under the 

 Fishery Board for Scotland).— No. X. By Prof. M'lNTOSH, 

 M.D., LL.D., F.E.S., &c. 



[Plate VIII.] 



1. On Abnormal Hydromedusae. 



2. On the Occurrence of the Ctenophores throughout the Year. 



3. On a Heteropod (Atlanta) in British Waters. 



1. On Abnormal Hydromedusae. 



Two examples of abnormal Hydromedusse having precisely 

 similar structure were procured by the midwater-net in 

 August 1886 in St. Andrews Bay. They occurred amidst 

 swarms of Thaumantias, Bougainvillia, Oceania, Stomo- 

 brachium, Cyanea, Aurelia, Pleurobrachia, Beroe, and other 

 forms. A brief description of these was communicated to 

 the Birmingham Meeting of the British Association the same 

 year *, it being pointed out that so far as specific characters 

 were present they seemed to be abnormal forms of Forbes's 

 Thaumantias melanops. The latter, however, was only half 

 an inch in diameter, whereas both of the specimens described 

 were about 5 inches in diameter. 



These large examples (Plate VI LI. fig. 1) were readily 

 distinguished by the presence of a simple pale cross of the 

 reproductive bands along the radial canals, the bands, more- 

 over, meeting in the centre of the disk, which was devoid of 

 a manubrium. The disk had the ordinary shape, viz. mode- 

 rately convex dorsally, somewhat flattened ventrally, and 

 presented no novelty in the microscopic structure of its 

 hyaline tissue. The margin is surrounded by a closely 

 arranged series of tentacles of considerable length, each taper- 

 ing from base to apex, and furnished with a single small 

 black pigment-speck at the base. The pigment-granules 

 show no special differentiation. Within the bases of the 

 tentacles is the velum. 



The reproductive bands, ff, begin a short distance within 

 the margin, and extend along the radial canals right across 

 the disk in each case, thus forming a conspicuous cross. More- 

 over, the uniformity of their diameter is one of the most cha- 

 racteristic features, no ordinary Thaumantias resembling them 

 in this respect, the nearest perhaps being Thaumantias pilosella. 



* Report Brit. Assoc. 1886, pp. 710, 711. 



