[Footnote 6: BEECHER Natural Classification of the Trilobites, p. 191, pretends that in Olenellus and Holmia real sutures »in a condition of symphysis» occur. He seems to deny the facial sutures and to accept as »real sutures» the »internal sutures» described by HOLM in Olenellus Kjerulfi. It is highly doubtful if these interior elevated lines are to be regarded as sutures. They are indeed no sutures, but in reality elevated linear ridges, inclosing, as it were, narrow canals. The real sutures known, the facial sutures, never form elevated lines, be it on the outside of the head or on the inside. Probably these lines are derived from some now unknown interior organization and it may be fit to remind of the somewhat similar though more numerous linear canals of the branchiæ on the interior surface of the great head shield of Apus. (ZADDACH De Apodis cancriformis Anatome ... pl. II fig. 1) or what HUXLEY (Anatomy Invertebr. Animals p. 281) calls the convoluted »shell gland» in the carapace. A quite different structure is what I suppose to be the incipient facial suture in Holmia.] [Footnote 7: WALCOTT in 10th Ann. Rept. U. S. Geol. Survey, pl. 82 f. 2.] [Illustration: Larva of Olenellus asaphoides, chiefly according to FORD and WALCOTT.] 1. Hypothetical figure, based on fig. 2. represents a stage preceding the next. There are three pair of pleura and three segments of the rhachis. 2. Copy of WALCOTT'S fig. 5 in »Bullet. 1886», pl. XVII. Three pleura at right, two at left, the larger formed through fusion of the two hindmost in fig. 1. 3. Copy of WALCOTT'S fig. 6: the second and third pleura have been united into one. 4. Hypothetical figure as a further development, following upon the stage represented in fig. 3. Fusion completed between all posterior plectra (2, 3, 4), thus forming a single large pleuron composed of the three mentioned. 5. Copy of FORD'S fig. 1, pl. IV, Amer. Journ. Science 1877, enlarged to the same size as Walcott's. First sign of the pygidium. 6. Copy of FORD'S fig. 2 (1877) enlarged. The pygidium has been added to the head shield. 7. Copy of FORD'S fig. 3, enlarged. 8. Copy of FORD'S fig. 1 page 251 in Am. Journ. Oct. 1881, enlarged. 9. Copy of FORD'S fig. 2 page 251. Amer. Journ. Science 1881. Slightly enlarged. 10. Copy of FORD'S figure 5, pl. IV, Amer. Journ. Sci. 1877, slightly enlarged. Already LINNARSSON observed the second ridge as he tells in a paper where he describes Olen. Kjerulfi for the first time.[8] It has been called an »ornamental» 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 Olenellidæ, at least not in their adult stage. [Footnote 8: Öfversigt Vet. Ak. Förhandl. 1871, tafl. XVI fig. 1.]