720 HISTORICAL GEOLOGY. 



Limuloids, appeared in tlie Silurian. They existed through the Carbon- 

 iferous era, and under more compacted forms have been continued to the 

 present time, four species now representing the genus Limulus, one North 

 American, and three East Asiatic and East Indian. The Carboniferous 

 genera Belinurus and Prestwichia represent, under an adult form, rather 

 closely, the young of the modern Limulus ; and Cycbis Packard considers 

 as representing a still younger embryonic stage of Limulus. 



6. Crustaceans. — It is stated on page 526, that Trilobites had their culmi- 

 nation in number of genera, and in number, size, and grade of species, in the 

 Lower Silurian. They continued, with few new genera, but under many new 

 species, in the Upper Silurian, and appeared under some extravagant spiny 

 forms during the Devonian ; but afterward, in the Carboniferous era, the 

 species were few and simple, only a score being known. The number of new 

 Carboniferous genera yet found is only two, and these are closely related to 

 the Devonian Proetus. Here the type ends. 



No other subdivision of Crustaceans appears to have passed its culmina- 

 tion in Paleozoic time excepting that of the Ostracoids, or the bivalved 

 Crustaceans (page 525). 



The Cirriped or Barnacle tribe, a degenerate group, derived from some 

 family of Ostracoids, as remarked on page 421, and one of the lowest stages 

 of Crustacean life, appeared as early at least as the Lower Silurian. 



Other tribes of Crustaceans continue to expand. True Isopods make 

 their appearance as early as the Devonian, and probably in the successional 

 line of the Trilobites. The Decapods are represented by Macrurans (or 

 Shrimps) in the Devonian, and by Brachyurans (Crabs) in the Carboniferous. 



Trilobites and many of the so-called Phyllopod Crustaceans are examples, 

 as has already been stated, of multipHcate forms, or those having an excessive 

 number of segments and members. The Early Cambrian Protocaris of 

 Walcott (page 474) is a good example of a multiplicate, Apus-\ike Phyllo- 

 pod, precursor of the true Decapod type. But normal numbers in segments 

 exist in some of the "Phyllopods," even those of the Cambrian, the abdominal 

 segments being reduced in number to six, the normal number in the Crusta- 

 cean type, and in the same Phyllopods the thorax also has apparently its 

 normal number of body segments ; in which case they are not multiplicate, 

 unless in legs, and these are not in sight in the fossil specimens. With 

 the appearance of Tetradecapods and Decapods in the Devonian, typical num- 

 bers, as to body segments and limbs — that is, for all parts of the structure 

 — have full expression; for the Isopods appear to be (in view of the 

 researches of Walcott, Matthew and Beecher) essentially non-multiplicate 

 Trilobites. 



7. Derivation of Limuloids and Crustaceans. — As has been suggested by 

 Lankester (and is recognized on page 423), it is probable that all the Articu- 

 lates are successional to the Rotifers. There is reason for believing, further, 

 that the type of Annelids, that of Crustaceans, and probably that of Limuloids, 

 had their independent Eotifer origin. 



