32 | Phillipsia | 5 | -- | 33 | Platymetopus | 1 | -- | 34 | Pliomera | 1 | -- | 35 | Proetus | 3 | -- | 36 | Ptychoparia | 1 | -- | 37 | Ptychopyge | 4 | 2 | 38 | Schmidtia | 1 | -- | 39 | Trochurus | 2 | -- | | +===========+=====================+ | Total | 136 | 36 | [Footnote 42: To these have here been added two Bohemian species P. bohemicus and P. spinosus omitted above at page 64.] We have thus 136 species of 39 genera in which hypostomic maculæ have been found and only 36 species, in which it has been possible to study the structure of the maculæ through sections. Common for a great number of maculæ in various groups, whether they show any organic structure or not, is the excessive thinness of their shell in comparison with that of the surrounding hypostoma. This is also in accordance with the tenuity of the cephalic eyes in relation to the test of the cheeks. The structure which characterizes the macula as a visual organ, although often so rudimentary, is not in all instances spread through the whole substance of the macula. This structure has in several species been restricted to a narrow circumference of the body of the macula, to its interior apex as in Bronteus and Proetus. Only in the Asaphidæ, in Illænus and Lichas the entire macula shows this structure. Perhaps, to judge by certain indications in Bronteus, once in a larval or preceding stage of evolution the whole surface of the macula was also in that genus covered with lenses, which have been reduced. The different groups in which the examined genera may be divided are the following seven, showing the remarkably great diversity of these organs. There is even in the same genus so great a variability that species with structure in the macula occur along with those devoid of any structure or also, as in Lichas, with a different structure. It must, however, be remembered, that the species of such genera may not be coeval. _Group 1._ In the sectioned macula there is no trace whatever of any structure. The test of the macula considerably thinner than that of the hypostoma. According to the affinities of the genera they may be subdivided as follows. _a_ Bumastus, Dysplanus. _b_ Nileus, Symphysurus, ? Ogygia. _c_ Calymmene, Homalonotus. _d_ Chirurus pro parte.