138 GEOLOGICAL DISTRIBUTION. 



Similarly, in the case of the Cephalopoda dibranchiata, the Belem- 

 nitidse are succeeded by forms more nearly resembling the cala- 

 maries, or cuttle-fishes (Teuthidse), of to-day, whose remains are 

 found already in the Jurassic deposits (Onychoteuthis, Teuthop- 

 sis, Belemnosepia). With the beginning of the Tertiary period* 

 we note the final disappearance of the varied group of the Ammoni- 

 tidse, and with them the last traces of all but one of the lower 

 or four-gilled order of cephalopods. The single exception is the 

 Nautilus, which, as a persistent type, almost unaltered from the 

 Silurian to the present period, alone survives to contest the seas 

 with the members of the higher or dibranchiate order. 



The predominant Mollusca of our modern seas, the Gasteropoda 

 and the Acephala, were but feebly represented in the seas of the 

 Cambrian period; but it seems not improbable that some of the 

 earliest forms e.g., Capulus, Pleurotomaria, among the snails 

 belonged to types absolutely identical with those living at the 

 present time. It is not until we have completely passed over the 

 Paleozoic era, which, so far as the Mollusca are concerned, may be 

 said to constitute the age of the Brachiopoda, that these two orders 

 of shell-fish attain any special significance. From the beginning of 

 the Mesozoic era onward they steadily crowd the deposits with their 

 remains, until, finally, with the Tertiary formations, and the forma- 

 tions succeeding these, they constitute the most characteristic and 

 most important invertebrate landmarks to the geologist and paleon- 

 tologist. 



It has been contended, and with apparent force, that the irregu- 

 lar appearance in time of the Mollusca i. e., the almost simultane- 

 ous introduction of forms belonging to both the lowest and the 

 highest orders, and the final supremacy in the existing seas of the 

 type of the Acephala, a group of mollusks inferior in organisation 

 to the Cephalopoda, the Gasteropoda, and the Pteropoda is in- 

 compatible with the doctrine of evolution, which^ as argued, re- 

 quires for its confirmation the introduction first of the lower forms, 

 the development from these of the more advanced, and, ultimate- 

 ly, the appearance of those that are most perfect or specialised in 

 structure. It must be recollected, however, that, as far as the al- 

 most simultaneous introduction of lower and higher forms is con- 



* A species from the Lower Tertiary of California. 



