Sir N. Yerinolojf — Diatoin EartJt in California. 273 



Simbirsk, Mors, and Oran, have species of their own ; thus the 

 Russian deposits of Simbirsk contain chiefly Dictyocha triommata 

 Kutz. 



Dictyocha,^ a genus estabhshed by Ehrenberg, was for a long time 

 thought to be a diatom ; later it was included by Haeckel in the 

 Radiolaria, but now it is proved to be a Silicoflagellate. Its species 

 are distinguished mainly by their geometrical outlines and the 

 number of horns, which vary greatly. The species Dictyocha fibula 

 and Dictyocha (or Distephamis) speculum are living at the present day ; 

 as stated by Sir John Murray, they are met with in the plankton 

 of North European coastal waters. 



Besides Dictyocha, the Lompoc deposit contains also specimens 

 of Actiniscus Sirius Ehr., but they are not dominating like those 

 of Dictyocha. 



The second main group of siliceous organisms in the Lompoc 

 deposit is that of the diatoms. These are all pelagic and nearly all 

 discoid, with only a very few gonoid forms. The Pseudo-Raphidese 

 are represented by several species of Thalassionema, also by 

 Baphoneis amphiceros, var. Californica, and by a species of 

 Tahellaria, which, however, is scarce. Of the so-called Pleonemic 

 diatoms, only one species of Chcetoceros has been found, namely 

 C. incurvum Bail., but the deposit undoubtedly contains many 

 Test-spores of Chcetoceros, such as Dicladia, Syndendrium, and 

 Hercotheca. The usual absence of Chcetoceros and Rhizosolenia 

 in fossil deposits is a fact which requires explanation, these genera 

 being so common in planktons. Why, while so common and widely 

 distributed, do their remains appear so rarely in the fossil deposits ? 

 Several explanations have been suggested. In the first place, 

 Chcetoceros and Rhizosolenia are very delicate organisms, easily 

 destroyed by acids in the preparation of the slides. Secondly, they 

 are neritic forms, thriving rather in coastal waters than in the high 

 seas. Thirdly, it is an undoubted fact, that those diatoms which 

 live in colonies and possess therefore special arrangements to 

 augment their floating surfaces and floating capacities, rarely can 

 fall down after death to the bottom of the ocean otherwise than as 

 separate individuals. This is the reason why colonies of diatoms 

 are, as a rule, so rarely met with in oozes and deposits in general. 

 Indeed, it may be supposed that colonies of diatoms, even after 

 death, continue to float about in the waters as dead plankton. No 

 Raphideae have been found in the Lompoc deposit, except one 

 species of Cocconeis. 



All the diatoms in Lompoc are undoubtedly northern forms ; 

 indeed, very similar to those usually common in European seas. 

 A few more or less southern species have been observed, but they 



^ An excellent monograph of the genus Dictyocha has been published by 

 A. Borgert, " Uber die Dictj'ochiden " : Zeitsch. Wiss. Zool., Leipzig, vol. li, 

 1891. 



VOL. LVII. — NO. VI. 18 



