The literature on the subject of fossil algae _versus_ inorganic and animal markings is too extensive and too wearisome to consider in a short summary; the student will find a sufficient amount of such controversial writing—with references to more—in the works quoted below[231]. In the Stockholm Museum of Palaeobotany there is an exceedingly interesting collection of plaster casts obtained by Dr Nathorst in his experiments on the manufacture of fossil ‘algae,’ which afford convincing proof of the value and correctness of his general conclusions. The pressure of the hand on a soft moist surface produces a raised pattern like a branched and delicate thallus. The well-known _Oldhamia antiqua_ Forbes and _Oldhamia radiata_ Forbes[232], from the Cambrian rocks of Ireland may, in part at least, owe their origin to mechanical causes, and we have no sufficient evidence for including them among the select class of true fossil algae. Sollas[233] has shown that the structure known as _Oldhamia radiata_ is not merely superficial but that it extends across the cleavage-planes. _Oldhamia_ is recorded from Lower Palaeozoic rocks in the Pyrenees[234] by Barrois, who agrees with Salter, Göppert and others in classing the fossil among the algae. The photograph accompanying Barrois’ description does not, however, add further evidence in favour of accepting _Oldhamia_ as a genus of fossil algae. The burrows made by _Gryllotalpa vulgaris_ Latr., the Mole-cricket, have been shown by Zeiller to bear a close resemblance to a branch of a conifer in half-relief (fig. 30, 4), or to such a supposed algal genus as _Phymatoderma_[235]. In fig. 30, 1, we have what might well be described as a fossil alga. This is merely a cast of a miniature river-system such as one frequently sees cut out by the small rills of water flowing over a gently-sloping sandy beach. A cast figured and described by Newberry as an alga, _Dendrophycus triassicus_[236], from the Trias of the Connecticut Valley, is practically identical with the rill-marks shown in fig. 30, 1. The cracks produced in drying and contracting sediment may form moulds in which casts are subsequently produced by the deposition of an overlying layer of sand, and such casts have been erroneously referred to algal impressions[237]. Dawson[238] has figured two good examples of Carboniferous rill-marks from Nova Scotia in his paper on Palaeozoic burrows and tracks of invertebrate animals. [Sidenote: RECOGNITION OF FOSSIL ALGAE.] [Illustration: FIG. 31. _Chondrites verisimilis_ Salt. Wenlock limestone, Dudley. From a specimen in the British Museum (V. 2550). Slightly reduced.]