Tuzoia (pl. 33, fig. 2), also approximates in pattern and size that of Nebaliopsis typica Sars.[1]
The merostomes Sidneyia,[2] Amiella,[3] Habelia (pl. 29), Molaria (pl. 29), and Emeraldella (pl. 30), have a compact cephalic shield and well-defined thoracic and abdominal segments and telson that are similar in character to the dorsal shield of the trilobite.
Labrum.—The labrum or hypostoma of Sidneyia[2] Emeraldella (text fig. 8, p. 204), and Marrella (pl, 26, fig. 2) is clearly shown in a number of specimens, also that of Burgessia (pl. 27, fig. 2), but in the other species it has not been seen nor has the lower lip (labium or metastoma) been observed in any species.
Segmentation.—The following table gives the number of cephalic, thoracic, and abdominal segments so far as known. The eyes are considered as representing a segment, which gives six segments in the cephalic region.
Table of Cephalic, Thoracic, and Abdominal Segments.
| Genera. | Cephalic segments. |
Thoracic segments. |
Abdominal segments. |
| BRANCHIOPODA. | |||
| Opabinia | ? | 16 | 1 |
| Leanchoilia | 4¹ | 9 | ? |
| Yohoia | 6 | 8 | 4 |
| Bidentia | ? | 11 | 1 |
| Naraoia | 6² | 17-19 | 3 |
| Burgessia | 6 | 8 | 30+ |
| Anomalocaris | .. | ? | 12+ |
| Waptia | 6 | 8 | 6 |
| MALACOSTRACA. | |||
| Hymenocaris | 6 | 8 | 6? |
| TRILOBITA. | |||
| Marrella | 6 | 24 | 1 |
| MEROSTOMATA. | |||
| Sidneyia | 5 | 9 | 3 |
| Habelia | 6?² | 11 | 2 |
| Emeraldella | 3 | 11 | 3 |
| Molaria | 6² | 8 | 2 |
¹ All that have been seen on imperfect specimens.
² The eyes are considered in this table as representing a cephalic segment.
In the table the telson has been included as an abdominal segment and the caudal rami are considered as attachments of the terminal segment.
Appendages.—So far as can be determined from the specimens now available for study the normal number of cephalic appendages
- ↑ Challenger Rept., 1887, Vol. 19, Pt. 56, pl. 3, figs. 1, 5, and 6.
- ↑ 2.0 2.1 Smithsonian Misc. Coll., Vol. 57, 1910, pls. 2-7.
- ↑ Idem. pl. 5.
