94 BRITISH PALAEOZOIC PHYLLOCARIDA. 



B. Shield-like Phjllopodom^s Carapaces sutured along the Bach} 



Before we proceed with the comparative descriptions, we may remark that a 

 few Httle fossils," similar to those about to be described, were noticed long ago by 

 palaeontologists, before the Crustacean characters of the latter were recognised ; 

 and their general likeness to the opercula of Ammonites ^ led some observers to 

 suggest that they may have belonged to Ooniatites, an " Ammonitidal " 

 Cephalopod found occasionally in strata of the same formation (Devonian) as that 

 in which certain of these Aptychus-like fossils occur.* Many of the real 

 Crustacean species, however, occur in beds in which Guniatites are unknown. 



Of the Phyllopodous forms under consideration we have some, like Discino- 

 caris, which could not, on account of their shape in general, and the presence of 

 the frontal piece in particular, have belonged to any Cephalopod. Next we have 

 a large series of forms which occur in beds wherein no Ooniatites have been found ; 

 and some occur in beds containing Goniatites, though their outlines do not 

 correspond exactly with the apertures of the shells of such Cephalopods. 



As other Phyllopods, such as Esfheria, are imbedded in Devonian rocks, it is 

 not strange that these Phyllocarida should be there also. 



Whilst, however, we are far from denying that some forms, now associated 

 with undoubted shield-bearing Phyllopoda, may hereafter be shown to be Molluscan, 

 we are certain that some have no relation to MoUusca. 



"We are the more strengthened in our opinion of the affinities of the Palaeozoic 

 Crustacean shields, because the ornamentation often agrees with that of known 

 Phyllopod carapaces, both in the minute, ridge-like, concentric lines of growth, 

 and, in some cases, in the delicate surface-ornament between them. 



Another objection to the supposed Aptychus-nature of many of these circular 

 and ovate shields arises from the fact that they were not originally flat discs or 

 plates, as may be seen by examining a series from various localities. 



Thus Discinocaris Browniana was in some degree convex, with a low conical 

 apex ; Asjndocaris triasica was evidently conical, as may be seen by the split state 

 of the outer rim, caused by the flattening of the whole shield ; others, as Lisgo- 

 caris Lutheri, had elevated sub-conical carapaces. ApUjchopsis not unfrequently 



1 Some Phyllocarida {Discinocaris and Aptychopsis) placed among Entomostraca were referred to 

 in Part I of this Monograph (1888), pp. 4 and 8. 



2 Aptychus Imvigatus, Goldfuss, 1832; A. vetustus, d'Arch. and de Vern., 1842; Aptychi of 

 Goniatites, von Keyserling, 1846 ; Aptychus dubius, F. A. Eomer, 1850 ; Aptychus of a Goniatite, 

 P. A. Eomer, 1850 (see 'Eeport Brit. Assoc, for 1884 ' (1885), p. 77. 



* Calcareous and bipartite, Aptychus ; eorneus and undivided, Anaptychus. 



* See, for instance, Dr. Dames' remarks in the ' Neues Jahrb. fiir Min.,' &c., 1884, pp. 275—279. 



