414 Prof. T. R. Jones and Dr. H. B. Holl on 



gyny is of the highest importance in connexion with the other 

 characters mentioned above, as opposed to the vermicular form 

 and white colour — characters common to most burrowing ani- 

 mals and plants. 



If this proves correct, there will only remain among the 

 Coelentcrata Lumbrici, Hirudines, Annelida, Echinodermata*, 

 Nematoidea, Tunicata, Bryozoa, Brachiopoda, Ctenophora, Rhizo- 

 poda, Spongiaria. It seems very doubtful whether these divisions 

 together would correspond to the three other subkingdoms. 

 The limits of the second subkingdom, Arthropoda, are much 

 litigated, as the lowest Crustacean does not show the same 

 degradation of the copulatory organs as the Fishes among Ver- 

 tebrata, and the Acephala among the Mollusca. I therefore 

 consider it probable that the subkingdom Articulata, chiefly 

 on account of the nervous system, ought to be reestablished 

 as it was defined by Cuvier. 



XLVII. — Notes on the Paleozoic Bivahed Entomostraca. No. VI. 

 Some Silurian Species (Primitia). Bv Professor T. Rupert 

 Jones, F.G.S., and Dr. H. B. Holl, F.G.S. 



[Plate XIII.] 



In the ' Annals of Natural History' for August 1855, September 

 1855, and April 1857 were published descriptions of some 

 Silurian Bivalved Entomostraca, comprising, among others, 

 Beyrichia of three types, — " simplices," " corrugatre," and 

 "jugosre." The first of these groups, the simple or unisulcate, 

 seems to us now to be deserving of generic distinction, since, 

 among a still larger number of forms, we find a persistent oc- 

 currence of the chief features, with a passage towards Leperditia, 

 by the complete loss of the furrow, rather than towards the two- 

 furrowed or real Beyrichw. 



We do not presume, however, that we hereby do more than 

 somewhat improve our classification of these necessarily obscure 

 Silurian Entomostraca, represented only by carapace-valves, 

 always minute and often variable in form and ornament, besides 

 being subject to alterations by pressure and by chemical change, 

 and rarely to be cleared of their matrix on all sides. In some 

 cases, too, we have had to be content with what we could make 

 out of casts and imprints. 



There remain, therefore, several difficulties in classifying these 

 little Bivalve Entomostraca — and especially since with the total 

 disappearance of the dorsal sulcus we do not seem necessarily to 



* According to Mr. "Williamson, closely allied to tlie Annelida (British 

 Association Report, 1857). 



