The Appendages, Anatomy, and Relationships of Trilobites

by Percy E. Raymond

The average of thoracic segments for the entire thirty-three genera is 10.5, of pygidial segments 5.9, a total average of 16.4. It will be noted that the thorax shows on the average less and the pygidium more segments than in the Lower Cambrian. If the Agnostidæ could be included, this result would doubtless be still more striking. Of the genera considered, Asaphiscus with 7-11 thoracic and 5-8 pygidial segments, Blainia with 9 thoracic and 6-11 pygidial, Zacanthoides with 9 thoracic and 5 pygidial, and Anomocare with 11 thoracic and 7-8 pygidial segments came nearest to the average. Only a few departed widely from it. The genera tabulated were Acrocephalites, Alokistocare, Crepicephalus, Karlia, Hamburgia, Corynexochus, Bathyuriscus, Poliella, Agraulos, Dolichometopus, Ogygopsis, Orria, Asaphiscus, Neolenus, Burlingia, Blainia, Blountia, Marjumia, Pagetia, Eodiscus, Goniodiscus, Albertella, Oryctocara, Zacanthoides, Anomocare, Anomocarella, Coosia, Conocoryphe, Ctenocephalus, Paradoxides, Ptychoparia, Sao, and Ellipsocephalus.

Enough genera of Upper Cambrian trilobites are not known from entire specimens to furnish satisfactory data. Excluding from the list the Proparia recently described by Walcott, the average total trunk segments in ten genera is 18, but as most of the genera are Olenidæ or olenid-like, not much weight can be attached to these figures.

For the Cambrian as a whole, the average for sixty-two genera is between 17 and 18 trunk segments, which is surprisingly like the result obtained by Carpenter from only twelve genera, and tends to indicate that it must be somewhere near the real average. If the 5 or 6 segments of the head be added, it appears that the "average" number of segments is very close to the malacostracan number 21. Genera with 16 to 18 trunk segments are Callavia, Protypus, Bathynotus, Crepicephalus, Bathyuriscus, Ogygopsis, Burlingia, Orria, Asaphiscus, Blainia, Zacanthoides, Neolenus, Anomocare, Conocoryphe, Saukia, Olenus, and Eurycare.

The order Proparia originated in the Cambrian, and Walcott has described four genera, one from the Middle, and three from the Upper. The number of segments in these genera is of interest. Burlingia, the oldest, has 14 segments in the thorax and 1 in the pygidium. Of the three genera in the Upper Cambrian, Norwoodia has 8-9 segments in the thorax and 3-4 in the pygidium; Millardia 23 in thorax and 3-4 in pygidium; and Menomonia 42 in thorax and 3-4 in pygidium. It is of considerable interest and importance to note that the very elongate ones are not from the Middle but from the Upper Cambrian.

Forty genera of Ordovician trilobites known from entire specimens were tabulated, and it was found that the range in the number of segments in the thorax and pygidium was surprisingly large. Agnostus, which was not included in the table, has the fewest, and Eoharpes, with 29, the most. While the range in number of segments in the thorax is 2 to 29, the range of the number in the pygidium, 2 to 26, is almost as great. A species of Dionide has 26 in the pygidium, while Remopleurides and Glaphurus have evidence of only 2. The average number of segments in the thorax for the forty genera was 10.15, in the pygidium 8.81, and the average number for the trunk 19.

Genera with just 19 segments in the trunk appear to be rare in the Ordovician, a species of Ampyx being the only one I have happened to notice. Calymene, Tretaspis, Triarthrus, Asaphus, Ogygites, and Goldius come with the range of 18 to 20. Goldius, with 10 segments in the thorax and (apparently) 8 in the pygidium, comes nearest to the averages for these two parts of the trunk. Goldius, Amphilichas, Bumastus, Acidaspis, Actinopeltis, and Sphærexochus are among the genera having 10 segments in the thorax, and there are many genera which have only one or two segments more or less than 10.

In most Ordovician genera, thirty-five out of the forty tabulated, the number of segments in the thorax is fixed, and the variation is in any case small. In four of the five genera where it was not fixed, there was a variation of only one segment, and the greatest variation was in Pliomerops, where the number is from 15 to 19. This of course indicates that the number of segments in the thorax tends to become fixed in Ordovician time. The variation in the number of segments in the pygidium is, however, considerable. It is difficult in many cases to tell how many segments are actually present in this shield, as it is more or less smooth in a considerable number of genera. Extreme cases of variation within a genus are found in Encrinurus, species of which have from 7 to 22 segments in the pygidium, Cybeloides with 10 to 20, and Dionide with 10 to 26. As the number in the thorax became settled, the number in the pygidium became more unstable, so that not even in the Ordovician can the total number of segments in the trunk be said to show any tendency to become fixed.

The genera used in this tabulation were: Eoharpes, Cryptolithus, Tretaspis, Trinucleus, Dionide, Raphiophorus, Ampyx, Endymionia, Anisonotus, Triarthrus, Remopleurides, Bathyurus, Bathyurellus, Ogygiocaris, Asaphus, Ogygites, Isotelus, Goldius, Cyclopyge, Amphilichas, Odontopleura, Acidaspis, Glaphurus, Encrinurus, Cybele, Cybeloides, Ectenonotus, Calymene, Ceraurus, Pliomera, Pliomerops, Pterygometopus, Chasmops, Eccoptochile, Actinopeltis, Sphærexochus, Placoparia, Pilekia, Selenopeltis, and Calocalymene.

Only sixteen genera of Devonian trilobites were available for tabulation, and it is not always possible to ascertain the exact number of segments in the pygidium, although genera with smooth caudal shields had nearly all disappeared. The number of segments in the thorax had become pretty well fixed by the beginning of the Devonian, Cyphaspis with a range of from 10 to 17 furnishing the only notable exception. The range for the sixteen genera is from 8 to 17, the average 11, the number exhibited by the Phacopidæ which form so large a part of the trilobites of the Devonian. The greater part of the species have large pygidia, and while the range is from 3 to 23, the average is 11.2. Probolium, with 11 in the thorax and 11-13 in the pygidium, and Phacops, with 11 in the thorax and 9-12 in the pygidium, approach very closely to the "average" trilobite, and various species of other genera of the Phacopidæ have the same number of segments as the norm. In every genus, however, the number of segments in the pygidium is variable, the greatest variation being in Dalmanites, with a range of from 9 to 23. The number of segments in the pygidium was therefore not fixed and was on the average higher than in earlier periods.