84 REPORT OF COMMISSIONER OF FISH AXD FISHERIES. 



Yar. DENSA. 



Fronds six to eiglit inches long, main axis densely clotlied witli very 

 numerons sliort branches. 



Common on stones near low-water mark along tlie whole coast. 

 The var. densa at Gloucester, Mass., 2Irs. Davis. 



A common species, recoguized by its tough, somewhat elastic substance, and 

 reminding one of bunches of small leather shoe-strings. When soaked in water it 

 gives out a large amount of slime, and is not easily mounted. To the naked eye it 

 resembles some of the forms of DicUjosiphon, but the microscopic structure is very dif- 

 ferent. The variety has been collected several times at Gloucester, but has not been 

 received from other localities. 



MESOGLOIA, Ag. 



(From fieoog, the middle, and yloLog, slimy. ) 



Fronds olive brown, gelatinous, filiform, branching; axial layer 

 composed of filaments rather loosely united into a solid mass, which 

 soon becomes fistulose ; peripheral layer of short horizontal filaments, 

 packed in a gelatinous substance ; unilocular sporangia oval, borne at 

 the base of peripheral filaments; plurilocular sporangia unknown. 



The old genus Mesogloia has been divided by modern algologists into a number of 

 genera. In the present instance we have kept in Mesogloia the species in which 

 the peripheral filaments are not transformed into plurilocular sporangia, and 

 have placed in Castagnea the species in which they are so transformed. The distinc- 

 tion between Mesogloia and Castagnea is artificial, because the plurilocular spo- 

 rangia of Mesogloia proper are unknown, and it is not impossible that they may be 

 formed from the peripheral filaments themselves, as in Castagnea. The development 

 of the fronds is not Avell known, and the genera founded upon the variations in the 

 mature fronds in the present group are iDlainly artificial. As regards its develop- 

 ment, M. divaricata resembles very closely C. virescens. From a disk-like expansion, 

 composed of a single layer of cells, which form spots on the substance upon which it 

 is growing, arise vertical filaments, which end in a hair such as is found in Ectocarpus 

 and other Phceosporece. The vertical filaments produce, usually only on one side, 

 fasciculated branches terminated l)y a hair, beneath which is a cluster of short 

 moniliform filaments. Besides these there arise, at a later period, rhizoidal filaments. 

 The mature fronds of the two species above named may be regarded as a collection of 

 filaments with a trichothallic growth, which have become twisted together and par- 

 tially united by means of the rhizoidal filaments, and whose fasciculated branches 

 constitute what, in the mature plant, seems to be a distinct cortical layer. In Cas- 

 tagnea virescens the separate filaments, with their lateral fasciculate branches, can easily 

 be isolated by dissecting the smaller branches, and the same thing can also be accom- 

 plished with Clwrdaria divaricata, although not so easily. The species of Mesogloia and 

 Castagnea should not be dried under too heavy pressure, and alcoholic specimens are 

 much better for study than those mounted on paper. 



M. DIVARICATA, Kiltz. {ChordaHa divaricata, Ag.; Phyc. Brit., PL 

 17; Ner. Am. Bor., Yol. I, PI. 11 a.) 



Fronds tufted, lubricous, six inches to two feet long, branching very 

 irregular, generally without a definite main axis; branches flexuous, ul- 

 timate branches very numerous, short, and divaricate, at first solid, 



