148 INVERTEBRATE PALAEONTOLOGY 



Cephalopods which appeared in the Ordovician, reached 

 its acme in the Palaeozoic, and has lingered with un- 

 distinguished persistence since the Permian. It is 

 interesting to note that this group of Tetrabranchs, the 

 earliest to arise, is still living; while the Ammonoids, 

 whose differentiation was deferred until the Devonian, 

 are utterly extinct. No types referable to Dibranchiata 

 (the only successful modern Cephalopods) are known 

 from the Palaeozoic. The most abundant Nautiloids 

 of the Ordovician and Silurian are " Orthoceracones " 

 straight, tubular shells with saucer-shaped septa. 

 Orthoceras (PI. x. fig. v.) is rare in the Ordovician, as, 

 indeed, are most other genera ; but, with its ornamented 

 relative Dawsonoceras, it is profusely distributed in the 

 Silurian. In the Ludlow series remarkable, probably 

 phylogerontic, forms allied to Gomphoceras are not un- 

 common. " Cyrtoceracones," resembling bent Orthoceras- 

 forms, were represented by the Silurian Ascoceras, while 

 Ophidioceras and Lituites showed some degree of en- 

 rolment preparatory to the " Nautilicone " condition. 

 Definitely " Nautilus "-like forms seem not to have been 

 developed before the Devonian. 



(K) ARTHROPODA 



With the probable exception of Protozoa, Arthropods 

 are by far the most abundant Invertebrates of the 

 present period. Marine and lacustrine plankton teems 

 with small Crustacea, while larger aquatic forms abound 

 in deep and coastal waters. On land and in the air 

 insects and arachnids, for all their normally small size, 

 can make other life miserable and precarious by their 

 insistent habits, although they excite wonder by the in- 

 tricacy and beauty of their structures. Most Arthropods 

 secrete "shells" of chitin, but in marine forms a fair 



