Fig. 82—Ceratiocaris papilio, One of the Fossil Phyllocarida. (From Lankester's "Treatise on Zoology," after H. Woodward.)
a, Traces of antennules; m, possibly mandibles; r, rostral plate
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Fig. 83—Pygocephalus cooperi, from the Coal-measures: Under-Side of a Female Specimen, showing the Overlapping Plates of the Brood-pouch. (From Lankester's "Treatise on Zoology," after H. Woodward.)
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The Malacostraca are more interesting from the point of view of palæontology than the other subclasses of Crustacea, since the evolution of the group appears to have taken place within the period covered by the fossil records, and it is possible to trace the course of that evolution—at least, in its broad outlines. It has already been pointed out that the most primitive of existing Malacostraca are the Phyllocarida (Nebalia and its allies), which are in several respects intermediate between the higher Malacostraca and the Branchiopoda; and it is interesting to find that fossils apparently belonging to the Phyllocarida are found far earlier than any of the other Malacostraca. In the Cambrian, and more abundantly in the Ordovician and Silurian, there are found Crustacea ([Fig. 82]) that resemble Nebalia in having a large bivalved carapace, with a movable beak-like plate in front, a projecting abdomen without conspicuous limbs, and a pair of large spines at the sides of the telson. Unfortunately, we have almost no knowledge of the structure of the limbs; but it can hardly be doubted that these very ancient Crustacea were allied to the existing Phyllocarida, and that they included the forerunners of the higher Malacostraca.
It is in the Carboniferous epoch, in all probability, that we must look for the origin of most of the existing orders of Malacostraca. In the rocks of this age in different parts of the world there have been found a number of undoubted Malacostraca, nearly all of the shrimp-like form which there is good reason to believe to be a primitive characteristic. Some of these (Pygocephalus—[Fig. 83]) have recently been shown to possess a brood-pouch formed of overlapping plates on the under-side of the thorax, and thus resemble the existing Mysidacea, which stand at the base of the Peracaridan series of orders. Others have a pair of strong side-spines near the tip of the telson, and in other ways resemble the recent Euphausiacea, so that they may have been primitive members of the Eucaridan series.