The Trilobites, after appearing in a great variety of generic and specific forms, and playing a most important part in their time, were not destined to continue beyond the Carboniferous period, and before that time they were beginning to give place to the Limuli, King-crabs, or Horseshoe-crabs, a few species of which continue on our coasts until the present time. In this limited duration the Trilobites present a strange contrast to certain shrimp-like Crustaceans, their contemporaries (the Phyllopods), which very closely resemble some still extant, and the same remark applies to swarms of little bivalve Crustaceans (Ostracods), which are still represented by hosts of modern species both in the sea and in the fresh waters. There is, however, a remarkable group of shrimp-like Crustaceans, represented in the modern world by only a few small species, which in the Cambrian age attained greater size, and constitute a very generalized type combining characters now found in lower and higher groups of Crustacea. _Hymenocaris vermicauda_ of Salter (Fig. 3) may serve to illustrate one of these primitive forms. [Illustration: Fig. 3.--_Hymenocaris vermicauda_, Salter. A Lower Cambrian Shrimp of generalized type. (After Salter.)] In point of fact, as Dr. Henry Woodward has shown in an able presidential address delivered to the Geological Society in 1895, at the base of the Lower Cambrian we still have several distinct groups of Crustacea; and if with some we were to hold them as traceable to one original form or to a worm-like ancestor, we must seek for this far back in those pre-Cambrian rocks in which we find no Crustaceans whatever. There is, it is true, no good reason to demand this; for whatever the cause, secondary or final, which produced any form of Crustacean in the Lower Cambrian, it might just as well have produced several distinct forms. Evolutionists seem to be somewhat unreasonable in demands of this kind, for any cause capable of originating a new form of living being, might have been operative at the same time in different localities and under somewhat diverse conditions, and may also have acted at different times. All imaginary lines of descent of animals are more or less subject to this contingency; and this may partly account for the great diversity in the lines of affiliation presented to us by evolutionists, which may in part have a basis in fact in so far as distinct varietal and racial forms are concerned, but may just as likely be entirely fallacious in the case of true species. In any case, in the lowest rocks into which we can trace Crustacea, we have already probably five of the orders into which their successors in the modern seas are divided by zoologists; and this is certainly a singular and suggestive fact, the significance of which we shall be better prepared to understand at a later stage of our investigation.