PECULIAR FORMS OF NAVICULA. 283 



in Pyrosoma is of the alternating type, whether in the adults, 

 or in the embryos, or in the nurse (cyathozoid). 2. The 

 circulation in the adults is lacunar and independent; in the 

 embryos it is vascular, and common to the young colony. 3. 

 The heart of the evanescent nurse presides over the circula- 

 tion of the compound embryos, Avhich is carried on by means 

 of a double cord, derived from the development of a canal, 

 and the vascular circle is closed in the nurse and in the last 

 of the embryos. 4. The heart of the nurse is replaced by the 

 hearts of the embryos, coexisting with the former during a 

 certain time, but independent of it. 5. The circvilation in 

 the embryos, produced by budding, takes place as in the 

 compound embryos, but is dependent on the mother, in place 

 of the nurse. 6. A lacunar circulatory system commences at 

 the same time that the vascular system begins to atrophy. 

 T. The pulsations of the heart are of variable duration. 



On some peculiar Forms of Navicula /rom the Sulu Archi- 

 pelago. By Rev. E. O'Meara, M.A. (With Plate XIII.) 



(Read at Meetiugof the Natural History Society of Dublin, March, 1872.) 



Great diversity of opinion has existed among the most 

 distinguished writers on the subject of the Diatomacese, as to 

 the question whether Ehrenberg's separation of Pinnularia 

 from Navicula is tenable or not. The ground on which this 

 distinction is made rests on the character of the stria% which 

 in Pinnularia are supposed to be costate, while in Navicula 

 they are moniliform. Kiitzing rejected this distinction, 

 Avhile W. Smith and Rabenhorst maintained its validity. 

 Ralfs in ' Pritchard,' p. 893, included the forms of Pinnularia 

 under the genus Navicula, and gives his reasons for adopting 

 this course as follows: — ''Were the costae always plainly 

 developed, as in Pinnularia nobilis and its allies, no difficulty 

 could occur in determining the genera, but in many of the 

 more minute species it is often very difficult to distinguish 

 between strise and costoe. We have not admitted Pinnularia 

 here, partly for the reason just given, but principally because 

 we cannot decide to which genus a large number of Ehren- 

 berg's species should be referred." The existence of tlie 

 distinctive characteristic is here admitted, but the genus 

 founded on it rejected on account of the difficulty of applying 

 it in many cases. 



