33 



phylum into two classes. Six pairs of names 

 have been proposed for these classes, of 

 which Inarticulata and Articulata are fa- 

 vored in most recent works. The Inarticu- 

 lata are usually divided into two orders, 

 and the Articulata were formerly divided 

 into two or three orders. There is no gen- 

 eral agreement on the orders of Articulata, 

 and the opinion of Hyman (1959) and 

 others that there is no satisfactory classifi- 

 cation is accepted here. However, one of 

 the older orders is still acceptable to pale- 

 ontologists, and there is no satisfactory 

 method for combining the remaining two. 

 The older division into three orders is 

 therefore followed, until a clear alternative 

 is available. 



MoUusca. Only one recent American 

 work is known to list seven classes of Mol- 

 lusca as is done herein. There is difference 

 of opinion among modern workers only as 

 to the status of the groups sometimes re- 

 ferred to as Isopleura. Many texts have 

 omitted the fossil Monoplacophora and 

 treated the Solenogastres as an incertae 

 sedis group. This leaves the Amphineura 

 or Placophora as a fifth class. It also leaves 

 the classification of the phylum incomplete 

 and therefore unsatisfactory. 



In volume I of the Treatise of Inverte- 

 brate Paleontology, Yonge (1960) dis- 

 cusses the features of these groups and con- 

 cludes that there are seven classes. This 

 arrangement is followed also in the Traite 

 de Zoologie and is accepted herein. 



Monoplacophora. Until 1957 this 

 class was known only as fossils. Its divi- 

 sion into three orders is taken from the 

 Treatise (I, 1960). 



Amphineura / Polyplacophora. There 

 are three substantially different classifica- 

 tions of this group among recent works. 

 These are: 1] orders Lepidopleurida and 

 Chitonida, by Thiele (1935), etc.; 2] or- 

 ders Eoplacophora, Mesoplacophora, Iso- 

 placophora, and Teleoplacophora, by Cot- 

 ton & Godfrey (1940); and 3] orders 

 Paleoloricata and Neoloricata, by Smith 

 (1960). There seems to be little direct cor- 

 relation between these systems. 



Smith's system is ostensibly based on 

 Pilsbry's early classification, modified by 

 paleontological data. It is unfamiliar to 

 neontologists, but it may be readily under- 

 stood when it is seen that all living chitons 

 are placed in the order Neoloricata (to 

 which many fossil forms belong as well). 

 It is adopted here as the best available 

 classification of a neglected group. 



Aplacophora. There appears to be no 



Notes on Subkingdoms and Phyla 



disagreement on the division of this class 

 into two orders. It is sometimes treated as 

 a subclass or even as an order, of Amphi- 

 neura or Gastropoda. 



Gastropoda. Nearly all recent writers 

 agree on the division of this class into three 

 subclasses, following Thiele (1931). The 

 first, Prosobranchia, has been widely di- 

 vided into three orders, but Cox (1960) in 

 the Treatise (I), has combined two of these 

 under the new name Caenogastropoda. 

 His new arrangement is followed here, al- 

 though in other respects Thiele's classifica- 

 tion is accepted. A recent classification by 

 Taylor & Sohl (1962) is not followed 

 here because it is not accompanied by jus- 

 tification at the ordinal level. 



Bivalvia / Pelecypoda. A considerable 

 variety of classifications are in use for this 

 class, •with little obvious correlation of 

 groupings. At the present time it appears 

 best to present the more common arrange- 

 ment of neontologists and also the custom- 

 ary paleontological one (in the footnotes), 

 until a single scheme has been accepted by 

 both groups of workers. The first of these is 

 the scheme of Lankester (1906) and most 

 later textbooks. The second is the scheme 

 of Cotton & Godfrey (1938) with the or- 

 ders raised to subclasses as by Cox (1960). 

 (Cox's more numerous orders may repre- 

 sent a more natural arrangement, but it is 

 not yet known whether they will be accept- 

 able to other workers.) 



Scaphopoda. Apparently no names are 

 available for the single class and the single 

 order. Therefore, Scaphopoda is here used 

 for all three levels, as they are coextensive. 



Cephalopoda. There is little agree- 

 ment among recent workers as to the sub- 

 division of this class, although many of the 

 subgroups are found in all schemes. The 

 arrangement followed here is that of Cot- 

 ton & Godfrey (1940) and many of the 

 older textbooks. The numerous nautiloid 

 "orders" of Flower & Kiimmel (1950) are 

 based largely on relative characters and 

 have not been clearly established as enti- 

 tled to ordinal rank. 



Sipunculoidea. The ten genera of this phy- 

 lum are not separated into classes or or- 

 ders. Separate names are not available for 

 the resulting single class and order. 



Echiuroidea. This phylum, which has of- 

 ten been placed in or appended to the An- 

 nelida, is divided into two classes as in 

 Boettger (1952). The class Echiurida is di- 

 vided into three orders as in Pearse 

 (1949). 



