8a CRETACEOUS PELECYPODA 



OsTEEiDM. The sixth and seventh orders represent the types of Pelecypoda with 

 only an anal opening and widely disunited mantle-margins ; both differ in habitat, 

 the former including marine and estuary, the latter fresh-water inhabitants. The 

 eighth order is principally characterized by entirely disunited mantle-margins and 

 usual total want of siphons ; the ninth by having two very unequal retractile mus- 

 cles, the anterior being small and sometimes nearly obsolete, the posterior much 

 larger. The last order is, strictly speaking, the only one in which the anterior 

 retractor is obsolete, or very nearly so, while the posterior large retractor assumes an 

 almost central position. However, it must be borne in mind that these remarks are 

 intended to apply only to the principal types, and that aberrant and representative 

 forms are everywhere to be met with, as I had already occasion to note. They are 

 the natural consequence of the course of development under different conditions. 



I shall conclude these remarks with a very brief and general notice regardino' 

 the distribution of the Pelecypoda in time and space. 



The Pelecypoda are found in all sedimentary formations from the earliest time. 

 In the Silurian period their number is, though smaller than that of the Brachio- 

 poda and Cephalopoda, still somewhat larger than that of the Gastropoda. In all 

 the palseozoic deposits the two lowest organized orders, the MTTILACEA and 

 OSTEACUA, prevail over the others; but most of the latter are also repre- 

 sented, though the four first ones mentioned in the above table barely by more than 

 a few species. In the mesozoic period, the number of the ZUCINACUA and 

 TULLINACUA distinctly increases; the OSAMA CU A make probably their first 

 appearance and rapidly diminish again towards the close of it ; the VHNEBAQBA 

 and JBKOLADBA also show their first characteristic forms, while the MYA CEA 

 attain a very large development and decrease from that time very sensibly. The 

 ABC ACE A appear steadily to increase from the earliest time; the VNIONACBA 

 are represented in the palaeozoic rocks only by a few species and are spar- 

 mgly found m the fresh-water deposits of the following formations; the F SOLA- 

 DACE A are rare through all the formation, but are already found in the pal^ozoics. 

 In the camozoic period all the orders are represented, and the geographical 

 distribution of the present time is clearly indicated; in fact this indication is 

 noticeable already during the cretaceous period, as I shall have repeatedly occasion 

 to remark. 



I^«°ki^S at the fauna of the present day, it is scarcely necessary to notice 

 tha^ by far the larger number of Pelecypoda is found in the tropical seas. Most 

 of the species 1 ye in the so-called Laminarian zone, many are also charac- 

 teristic littoral types, and various species are met with from low water to the 

 greatest depth (6-800 fathoms) in which MoUusca have been discovered. Special 

 locahies characterized by certain orographical conditions and climate, have, as 

 usuaUy their peculiar faunas. There are only few species which have a general 

 distribution, and for these (as in the case of many FHo.^r.,n^J special reasons exist. 

 I will give greater details regarding the distribution of the various genera, &c., 

 when treating specially of each family. 



