370 GENERAL SUMMARY TO 



with strongly plaited valves and a high median fold in the dorsal, and a corresponding 

 sinus in the ventral valve. Hinge-line curved, beak truncated, with a terminal round 

 foramen ; deltidium formed of two distinct plates, which limit the foramen below only 

 for a very short distance. Internally, the ventral valve with two strong hinge-teeth, 

 which are, however, not supported by dental plates. The dorsal valve bears a tolerably 

 large triangular hinge-plate, which is united on both sides by the deep dental sockets, 

 and is triangularly cut open in the middle up to the very apex of the valve, which also is 

 a little cut out. There is no cardinal process. On both sides of the median incision very 

 short curved crura take their origin, and proceed for a short distance in a slightly diverging 

 direction towards the interior of the shell. There is no median dorsal septum. . . 

 . All the internal partitions, which are characteristic for Bhynchonella, the dental 

 plates as well as the dorsal septum, are absent in Terebratuloidea. ... I cannot 

 positively assert that the beak has never had the shape of the beak of Bhynchonella, but 

 I can most positively say that I have never seen a specimen in which the beak was not 

 truncated." 



I am ready to agree with Dr. Waagen as to the propriety of dividing the family 

 Bhynchonellida into the three sub-families, Bhynchonellince, Camarophorince, and Penta- 

 mcrina. It is quite evident, therefore, that although we are gradually advancing towards 

 a more satisfactory classification, there is still a great deal more to be found out and 

 explained before the much desired end can be attained. 



Our tables show at a glance that the Tretctiterata, with the exception of Linyula, 

 Glottidia, Craniscus, and Actinocrania, were all essentially Palaeozoic forms, and that the 

 larger number occurred chiefly during the Silurian period ; Linyida, Discina, and Crania, 

 however, continuing to be represented with very similar shapes up to the present time. 



The families Terebratulinidce and Thecideidce have been classed in thirty-three genera, 

 and as far as we are aware ten have occurred in the Palaeozoic Rocks, eighteen in the 

 Neozoic, fourteen in the recent period, Waldhcimia alone having ranged from the Silurian 

 up to the present time, and two only having passed from the Palaeozoic into the Triassic. 

 (See pp. 353, 354.) 



The family Splriferida consists mostly of Palaeozoic forms, which are chiefly Silurian, 

 Devonian, and Carboniferous. Of the thirty-six genera quoted in our Table, thirty-two 

 occur in the Palaeozoic period, nine in the Mesozoic, and they disappear completely in 

 the lower half of the Jurassic. (See Tables on pages 354 and 355.) 



In the Bhynchonellidm we have admitted twelve genera. Of these, ten occur in the 

 Palaeozoic Rocks, two in the Neozoic and recent, Bhynchonella alone continuing to be 

 represented from the Silurian period up to the present time. The sub-families Camaro- 

 phorince and Pentamerina are known only in the Palaeozoic Rocks. (See Table, page 355.) 



The families Porambonitida, Orthisidce, Strophomenida, and Productidce may be said 

 to be all Palaeozoic, the doubtful three so-termed Lcptana from the Liassic period excepted. 



