52 D. J. SCOURFIELD OX THE EPHIPPIUM OF BOSMINA. 



margin of the original valves, there is a distinct increase in the 

 thickness of the chitin, and, further, that there is a narrow, 

 highly refractive band of chitin running somewhat obliquely 

 across each valve from near the anterior dorsal angle to within 

 a short distance of the posterior margin. It is the possession 

 of these lateral thickened bands of chitin which distinguishes 

 the ephippium of Bosmina from all homologous structures. 



If one of the lateral bands of chitin be still further examined 

 it will be seen that it probably represents a modified line of 

 hexagonal areas, forming part of the shell sculpture of the 

 unaltered valve. But all the cross partitions have been obliterated, 

 and the edges of the band have been smoothed so as to show but 

 little indication of the original polygonal arrangement. The 

 extremely minute granulation or pitting of the areas enclosed 

 by the shell markings has also disappeared, for the chitin of 

 the band seems absolutely homogeneous (Fig. 5). In continuation 

 of the band anteriorly, there is a slit in the valve which runs up 

 to the anterior margin near the anteiior dorsal angle. The edges 

 of this slit are normally in contact, but may be easily separated 

 by pressure. The slit seems to be produced by the falling out of 

 a number of pieces of chitin (Fig. 6), exactly in the same way as 

 I have described in the case of the line of separation along 

 the ventral edge of the ephippium and round the valve-margins 

 in Leydigia acanthocercoides (see " The Winter Egg of a Rare 

 Water-Flea," Journ. Q. M. C, Series 2, Vol. vii., p. 171). The 

 pieces of chitin no doubt represent modified hexagonal areas of 

 the original shell sculpture in the same line as the series which 

 produce the lateral band. In connection with this it may be 

 pointed out that the anterior end is often seen to be separated 

 from the rest of the band by a transverse line (Fig. 1). At its 

 posterior end the lateral chitinous band appears to end abruptly 

 in the ordinary shell markings. 



In Bosmina longirostris the lateral bands do not project very 

 much beyond the surface of the shell, but in a species (Fig. 8) 

 from Upper Lough Erne (probabJy Bosmina UUjehorgi Sars, 

 although this is perhaps only a variety of B. coregoni Baird), very 

 kindly sent to me by Mr. W. F. de Y. Kane, the projection of 

 the lateral bands is so pronounced that they deserve to be termed 

 i-idges (Fig, 9). The position of the bands in this case is also a 

 little different to what it is in B. longirostris. As will be seen 



