62 BULLETIN OF THE 



of the retina, and separates the remaining retinal surface into two parts, 

 one on either side of the stalk. In alcoholic specimens this median band 

 is readily visible with the aid of a hand lens, and a little closer scrutiny 

 shows that it is composed of six lines. These lines, of course, correspond 

 to the six rows of ommatidia previously mentioned. The smaller om- 

 matidia, on either side of the median band, are also arranged in lines 

 parallel to those in the band ; but, on account of their smaller size, the 

 lines formed by them are not visible with an ordinary lens. 



The smaller ommatidia in Goniodactylus are arranged upon the typi- 

 cal hexagonal plan (see the left half of Fig. 93, Plate VI II.). The 

 larger ones have a somewhat similar grouping, although the fact that 

 they are in six longitudinal rows rather obscures their hexagonal ar- 

 rangement. (See the right half of Figure 93, in which three rows, and 

 a part of a fourth, of large ommatidia are shown.) The hexagonal 

 arrangement is not disturbed, as might be expected, on the line which 

 separates the larger from the smaller ommatidia, but both kinds form 

 parts in a common system. That this is true can be seen from Figure 

 93, where it will be observed that the centres of any two small ommatidia 

 lying in the same vertical line are as far apart as the centres of the cor- 

 responding larger ommatidia. Moreover, as I have demonstrated by 

 actually counting the ommatidia of long parallel series, a vertical band 

 which contains twenty-five large ommatidia has the same length as one 

 composed of a corresponding number of small ones. The apparent differ- 

 ence in numbers at first sight presented by lines of the two kinds of 

 ommatidia is principally due to the fact that the larger ommatidia are 

 arranged in distinct rows, whereas the smaller ommatidia are so grouped 

 that the individuals in one row are slightly interpolated between those 

 of the two adjoining rows (compare Fig. 93). 



In Decapods the ommatidia are arranged either upon the hexagonal 

 or tetragonal plan. In the Brachyura, 1 as well as in three families of the 

 Macrura, the Hippidre, Paguridie, and Thalassinida:, 2 the arrangement 



1 The presence of hexagonal facets has been recorded in the following genera 

 of Brachyura: Port,n»is (Will, '40, p. 7) ; Ilia (Will, '40, p 7, Leydig, '55, p. 411) ; 

 Cancer; Maja; Carpllius (Frey una" Leuckart, '47, p. 204) : Herbslia, Dorippe, and 

 Lambrus (Leydig, '55, pp. 407, 410, and 411, respectively). This form of facet is 

 present only when the ommatidia are hexagonally arranged. Leydig ('55, p. 411) 

 states that the outline of each facet in Dromin Rumphii is square, but, as his 

 description clearly indicates, the facets are arranged upon the hexagonal plan. 

 As my own observations show, the ommatidia in Cardisoma Gvanhumf, Latr., 

 Cancer irroratus, Ray, and Gelasimus pugilator, Latr., are hexagonally grouped. 



- The outline of the corneal facets is stated to be hexagonal in the following 

 genera: Pagurus (Swammerdam, '52, p. 88, Cavolini, '02, p. 130, Milne-Edwards, 



