14 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



Already Linnarsson observed the second ridge as he tells in a paper where he 

 describes Ölen. Kjerulfi for the ■ first time. * It has been called an »ornaraental» spine, 

 but in the following we shall learn what it really is. This ridge in connection with its 

 spine has not been observed in any other of the Olenellidae, at least not in their adult stage. 



Thanks to the discovery of very early larval stages of the American Olenellus 

 asaphoides, which Ford 2 and Walcott 3 have described and figured, we can through 

 combination of these decipher the development and signification of the facial ridges. To 

 facilitate my explanation I here join a series of cuts from the earliest stage to the more 

 developed, with addition of two schematic stages to complete in a certain degree the 

 series of Walcott and Ford. 4 See figures p. 13. 



The figure 1 has been hypothetically composed as a deduction from tig. 2, which 

 presupposes an earlier stage of development like that in fig. 1, when there existed three or 

 four pair of lateral appendages in the larva. This then consists of a central portion of tive 

 segments. The large anterior crescentlike segment does not, however, show any distinc- 

 tion between a central part and lateral appendages, it is nearly as large as the three next 

 taken together and its backwards bent side horns embrace the next two posterior segments 

 and attain with their narrow pointed tips the back of the fourth segment. The central portion 

 consists of five segments, when the somewhat not well definite posterior marginal segment 

 is taken in account. Each of these segments excepting the fifth one has lateral appendages, 

 those of the second and third segment being quite as broad as the central part and bent 

 backwards in a curve ending in a small pointed spine. The lateral appendages of the 

 fourth segment are largest of all, more than double the length of the two next in front, 

 triangulär and standing out beyond the posterior börder of the shell as a broad spine. 



In fig. 2 a great change has set in. There is no distinction between the lateral 

 appendages of the third and fourth segments at left. These two have been fused to- 

 gether, they have united, so as to make the left triangulär spine look larger than it was 

 originally. The appendages of the right side are still in the same state as before. But this 

 fusion of the lateral appendages also takes place in another direction, as shown in another 

 specimen (tig. 3), so that the second and third appendages on both sides coalesce into one 

 piece. Now it is easy to imagine that at last a complete fusion has set in between all 

 lateral nppendages and that instead of the original three on each side, there is only one large 

 l>ioc<\ reaching beyond the shell as a broad spine, as represented in the hypothetical figure 4. 



In the figure 3 a progressive change is also seen in the transformation of the first 

 central segment. From occupying the whole foremost space of the shell it has been lesse- 

 ned in size, more distantiated from the anterior börder of the shell and rounded off, more 



1 Öfversigt Vet. Ak. Förhandl. 1871, tafl. XVI fig. 1. 



2 In American Journ. of Science 1877 p. 265 and 1881 p. 250. 



3 Hullet. U. S. Geol. Survey N:o 30, 1886, pl. 17 fig. 5—6. -- Tenth Annual Rept, U. S. (ieol. Sur- 

 vey, 1888—89, printed 1890, plate 88, fig. 1, 1 a. 



1 In bis paper on The larval stages of Trilobites» p. 175 Bkecher gives a new figure (f. 6), original 

 from FORD's collection, of the larva of 01. asaphoides, hut it is so sketchy that 1 cannot with certainty make 

 out what it means. I cannot agree with him when he speeks of free cheeks and eyes in these and he is com- 

 pletely vvrong when he savs that the outer pair of spines helong to the free cheeks etc. (p. 176). 



