Fossil Ostracoda. 165 



many so-called species the ornament passing from one to the 

 other in a very perfect series. Still, this character, considered 

 with the ventral and end aspects of fossil forms, affords a safe 

 basis on which to classify and keep within bounds the species of 

 the fossil Ostracoda. 



Hence it is very important that the student of fossil Ostracoda, 

 not being able to examine the animal as a guide to his researches, 

 shall, before determining species, make definite and exact 

 drawings of the side, ventral, and end views of the carapaces he 

 is examining ; and not only this, but these drawings should be 

 all made from one individual, so as to ensure coiTCct and faithful 

 observation and outlines. Further, it is necessary to note the 

 relative size of the right and left valves, the position and form 

 of the teeth, and, if possible, to determine the shape and 

 characters of the muscle-spot. Then, and only then, can the 

 identification of species become possible. It is easy enough for 

 any one to become acquainted with the characters of the genera 

 of any particular group of organisms ; but it requires long study 

 and very careful observation to enable the student to have a 

 definite understanding of the differences which combine together 

 to form a so-caUed species, especially amongst fossil forms. 



Now, with regard to the importance of the Ostracoda in the 

 geological series : palaeontologists cannot fail to be impressed with 

 the enormous abundance of the remains of the smaller organisms 

 in the sedimentary rocks, as compared with those of the larger 

 organisms. The Foraminifera, with the genera Nummulites, 

 Fusulina, &c., have, with a little cementing material, formed 

 hundreds of feet of strata. The Sponges, as lately shown by 

 Dr. Hinde, have largely composed many, if not all, of the thick 

 deposits of chert in the carboniferous period ; and the Ostracoda, 

 although we do not see such stupendous thicknesses of rock- 

 masses formed of their remains as we do with the Foraminifera 

 and the Spongida, still we owe considerable thicknesses of rock 

 at different periods of our geological history to the reproductive 

 energies and cast carapaces of these minute Crustacea. 



In early Silurian times the genera Beyrichia, Thlipsura, &c., 

 left very considerable quantities of their remains in the Wenlock 

 and other shales. In the Devonian rocks Ostracoda are rare ; 

 but the Kev. G. F. Whidborne has recently discovered some 

 Beyrichian forms near Torquay, one of which has been described 

 by Professor Eupert Jones under the new generic name of Kyam- 

 odes. Entomis serrato-stiiata of the Continent is the best known 

 Devonian form. In Carboniferous times the Ostracoda seem to 

 have reached the period of their greatest abundance, beds of 

 shaly limestone and carbonaceous shale being in some places 

 crowded with their remains. In a small piece of shale weighing 

 less than one ounce, which I lately examined, I obtained over 



c2 



