1 1 



As remarked 1. c, this is one of the most delicate species of the genus Lithothamnion 

 attached to hard objects. It sticks to different other calcareous algae, especially dead or stunted 

 ones, calcareous sediment, corals, dead shells of molluscs, or occasionally stones. When the 

 plant occurs on a level, or fairly level, substratum, the crust is smooth and generally has a 

 thickness of 30 — 60 y.. In this case it clings rather closely and often firmly to the substratum 

 (PI. I, fig. 1 — 2). Frequently, however, new crusts will be formed, one above the othei/, or 

 the primary crust will develop secondary incrustments. Such a complex of crusts may attain a 

 thickness of up to 300 [j.. The species is apt to being overgrown by other calcareous algae, 

 and is often itself attached to other species. A section of the plant with its substratum will, 

 therefore, now and then exhibit layers of two or several species growing one above the other 

 or even alternating with lower animals, e. g. Bryozoa. 



If the substratum is rough, or if foreign bodies have attached themselves to the primary 

 crust, the surface of the plant fmally becomes more or less uneven. The formation of new 

 crusts on fertile specimens, expanding over undissolved conceptacles of sporangia or cystocarps, 

 or the occurrence of the plant on other crustlike species which are sparingly branched, e. g. 

 stunted Lithothamnion fruticulosum f. clavulata, gives rise to the form which I have named 

 f. pseudoramosa. The form is certainly but slightly differentiated and ought perhaps not to be 

 kept distinct. However, as it is externally often rather diverging from f. typica, I classify it as 

 a separate form (PI. I, fig. 3 — 9). Besides it does not appear to adhere so intimately to the 

 substratum, but shows a tendency to partly detaching itself from the latter. 



As regards the structure of the species, vertical sections show that the hypothallium ') 



frequently consists of a few layers of cells whose lower anticlines are 



often but slightly converging to the substratum, sometimes not 



converging at all, and apparently never marked coaxial. The cells are 



elongated and may be up to about 25 f/., long, though they rarely exceed s 



12 — 18 u. The perithallium, issuing from the hypothallium and as =/^^Z)ODOOCDd 



. . JODddqdOD 



a rule taintly developed, is formed by cells which are partly roundish, ~]{~]( '1/' ]j ?r 1/ — 1/ — \r~)\~ 



partly square, 6 — 10 p.. in diameter, often, however, a little elongated, ~)(~]r~|( }r~)r~}f~)ni — ir 



with the longer diameter now in horizontal, now (and more frequently) "VTAOf - ) f~) OiOO DC 



in vertical direction, in the latter case being exceptionally up to 18 a. ^ r inr\nnO^ 



long and 10 a. broad ; fig. 3"). Flg ' 3 ' 



Lithothamnion siamense Fosl. f. typica. 



The reproductive organs hitherto known are sporangia and A. Vertical section of the crust; x 72. 



, • , -t-rr v • 1 1 t^i r /; - Part of the perithallic laver in ver- 



cystocarps which occur on dirterent individuals. 1 he tormer are ,, 



J 1 tical section; X 390. 



subprominent or almost superficial, 300 — 400 u. in diameter, when 



seen from above, at first convex, afterwards more or less flattened, with a thin roof, perforated 



1) The two kinds of tissue which form the thallus in most of the algae in question were indicated by SOLMS-LAUBACH as 

 Markstrang and Hinde. Areschoug used for the former the denomination of hypothallium^ and for the latter Rothpletz pvoposed the 

 name of perithallium. 1 have adopted the latter name in the same sense. As to the former, I also use the term of hypothallium for the 

 basal or rhizoidal layer, composed of rows more or less bent or coaxial in a number of crustlike fornis. If this tissue consists of only one 

 basal cell-row, as the case is in some other forms, I simply name it the basal cells or one-layered hypothallium. In attached, branching 

 forms there is frequently some difference in size and sometimes even in shape between the said hypothallium in the crustlike part of the 

 plant and the inner layers of radiating tissue in the branches. In describing the structure I therefore name the latter medullary hypothallium. 



2) All the figures showing the structure of the species are from undecalcified sections which have been made thin by grinding. 



