In reviewing the genera and species enumerated above, as to their number in which the structure of the macula has been observed, we arrive at the following results.
| Genera. | Number of species with maculæ. | Number of species in the maculæ of which the structure has been examined. | ||
| 1 | Acaste | 1 | — | |
| 2 | Acidaspis | 1 | ? | — |
| 3 | Asaphus | 18 | 7 | |
| 4 | Barrandia | 1 | — | |
| 5 | Bronteus | 9 | 5 | |
| 6 | Bumastus | 7 | 1 | |
| 7 | Calymmene | 4 | 2 | |
| 8 | Centropleura | 1 | — | |
| 9 | Chasmops | 1 | — | |
| 10 | Chirurus | 9 | 2 | |
| 11 | Ctenopyge? | 1 | — | |
| 12 | Dalmanites | 10 | 2 | |
| 13 | Dysplanus | 2 | 2 | |
| 14 | Encrinurus | 3 | 1 | |
| 15 | Griffithides | 1 | — | |
| 16 | Herpes | 2 | — | |
| 17 | Harpina | 1 | — | |
| 18 | Holmia | 2 | — | |
| 19 | Homalonotus | 2 | 1 | |
| 20 | Hysterolenus | 1 | — | |
| 21 | Illænus | 9 | 5 | |
| 22 | Lichas | 4 | 1 | |
| 23 | Megalaspides | 2 | — | |
| 24 | Megalaspis | 3 | 1 | |
| 25 | Nileus | 2 | 2 | |
| 26 | Niobe | 5 | 2 | |
| 27 | Ogygia | 3 | — | |
| 28 | Paradoxides[42] | 8 | — | |
| 29 | Peltura | 1 | — | |
| 30 | Phacops | 3 | — | |
| 31 | Phaëtonides | 1 | — | |
| 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 | ||
[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.
Group 2. The whole macula is of a spongious or irregularly polyedric structure, showing prisms in vertical sections. Its concordance with the structure of the deteriorated cephalic eyes or with the so called border zone is complete. If a supposition may be hazarded, I think that the spongious or reticulate structure in the maculæ is their real and original state, a lower stage of development of the visual organs, out of which the prisms in the cephalic eyes have been formed, and that the prismatic lenses in their decay, as seen in the Asaphidæ, reveal to us their original state and structure, and thus, as it were, return to the primary stage in the maculæ.
Asaphus, Isotelus, Megalaspis, Ptychopyge, Niobe, ? Megalaspides, ? Barrandia.