lACKAED.] FOSSIL ALLIES OF NEBALIA. 443 



as proceplialie lobes with eyes, all these parts not being developed in 

 the embryo Mysidae. 



But whatever may be said of the resemblances between ISTebalia and 

 the MysidcB at an early period after the nauplius stage has been dis- 

 carded, when we compare the later stage represented by Metschnikoff's 

 fig. 19 (our fig, Q^, in text) with the latest larval stage of Pseudomma 

 (see Sars's figure, our Plate XXXVIII, fig. 5), then we see that the 

 diagnostic ordinal characters of the Phyllocarida have declared them- 

 selves. There are to be seen in Nebalia the large movable rostrum, the 

 compressed pseudobivalvular carai)ace, the lack of maxillipedes, the 

 eight pseudophyllopod thoracic feet, four pairs of abdominal feet, out 

 of the six of the adult. On the other hand, in Mysis of the same stage, 

 the two pairs of maxillipedes are well developed, and the six pairs of 

 remarkably long thoracic feet (the first pair modified maxillipedes) are 

 l)resent. There is little to indicate that the Schizopods have descended 

 from a ISTebalia-like form, but rather from some accelerated zoea form ; 

 while, as we attempt in this essay to show, the Phyllocarida have had 

 no Decapod blood in them, so to speak, but have descended by a sep- 

 arate line from Copepod-like ancestors, and culminated and even began 

 to disappear before any Malacostraca, at least in any number, api)eared. 



II. — The Paleozoic Allies of ]!^ebalia. 



Having studied the anatomy and development of Ifebalia we are pre- 

 pared to compare it with a group of fossil forms which are scattered 

 through the older Paleozoic rocks from the lowest Silurian to the Car- 

 boniferous. In a brief article^ Mr. Salter, nearly twenty years since, 

 sketched out the characters and showed the relationship of Ceratiocaris 

 and a number of allied forms to Nehalia in the following paragraph : 



"Before the structure of Ceratiocaris was known, of which genus a 

 reduced figure is here given, the rostral portion of Peltocaris could not 

 have been understood. But a reference to the accomj)anying series of 

 wood-cuts will show that a tolerably broad rostrum, placed in the same 

 relative position, occurs in Ceratiocaris. In the recent I^ehalia it is fixed, 

 and in Bitliyrocaris and other genera it is perhaps yet to be discovered. 

 Again, Ceratiocaris, together with its movable rostrum, has a bivalved 

 shell, yet habitually keeps its valves half closed, as I learn from per- 

 fect specimens." 



Salter then enumerates the characteristics of the fossil genera, begin- 

 ning with Hymenocaris, which he considers the more generalized type, 

 and in the wood-cuts which we partly here produce shows the geologi. 

 cal succession of these genera, which also serves as a genealogical table- 

 He regards them as Phyllopods, associating Estheria and Apus, regard- 

 ing the latter as "the most comi)lete and decided form, and it is one of 

 the latest of the group, as it commences in the Trias." He also says : 

 "The links between these coal-measure forms and those of recent times 

 are many of them wanting ; but in Nehalia we have a good representa- 

 tive of the compact, shield-shaped form of Ceratio.caris, the two valves 

 soldered into one, and the rostrum attached, the eyes being still beneath 

 the carapace." It is evident from this that Mr. Salter regarded the fos- 

 sil genera he enumerates as allied to and as the ancestors of JSiebalia, 

 and as representatives of it in Pala30zoic times. He evidently adopted 

 the views of Milne-Edwards and others as to the Phyllopodous nature 

 of Xebalia. 



1 Ou Peltocaris, a new genus of Silurian Crustacea, by J. W. Salter, Quarterly Jour- 

 nal of the Geological Society of London, vol. xix, 1863, p. 87. 



