MICRASTERIAS DENTICULATA. 117 



occurs so rarely tliat when a reversed \-anety appears the supersti- 

 tious put upon it an extravagant value. In this country the 

 sinistral variety seldom appears in collections of conchology, and 

 seven or eight guineas have been given for this very uncommon 

 object. 



1Hote on tbe Binary Sub^Mvieion of flDicrae^ 

 teriae Scuticulata (Breb.j, 1Ralf6, 



By Stephen Helm, U.S.A.* 

 Plate X. 



ON Saturday, the 15th of June, at 10.45 P-m., I had the 

 good fortune to perceive a solitary specimen of Micras- 

 terias denticulata in a state of binary sub-division. 



It had then arrived at what I will call the five-lobe stage, i.e.^ 

 the lobe on each side of the connecting central one had already 

 divided, making five, which were as nearly as possible equal in size. 



The lobe numbered 2 (Fig. 2) soon gave signs of lateral en- 

 largement, and about 10.55 each top had a small but distinct 

 heart-like depression (Fig. 3). 



As the lobe became broader, this depression deepened until 

 11.30, when the division was completed, and well defined (2, 2^7, 

 Fig. 4). About 11.25 lobe number 3 (Figs. 2 and 4) on each 

 side evinced signs of enlargement; and at 11.35 the before- 

 mentioned heart-like depression was apparent. 



I had by this time become deeply interested in this simulta- 

 neous quadruple growth, when to my intense disgust and 

 disappointment, down came a Cyprts and gobbled up my Desmid, 

 and brought my observations to a summary conclusion. 



I should not have deemed these observations worthy of record, 

 but for the fact of their leading me to a conclusion somewhat 

 different from that of the description of this Desmid, given by 

 Carpenter and Hogg. 



These authors state that the divisions are made in the follow- 

 ing order: i, 3, 5, 7 and 13, and that it takes place in about three 

 and a-half hours. 



■* From "Journ. New York Micro. Soc. " 



