It may be worth while here to remind of the great similarities, whether analogous or homologous, in the formation of the superior surface of the head in the trilobites and the embryons and the newly hatched larva of Limulus. The latter have the head shield separated into five portions, partly through a facial suture which, as in Peltura, Dalmanites and others, forms a continuous line around the glabella. This suture divides the eye node in a similar way as in the trilobites, that is, sectioning it in two parts, of which one adheres to the central fixed cheeks and the other to the free cheeks.[17] And to judge by the figures of Kingsley the former, the whitish moiety is the first developed and sometimes for a while quite alone as the facial ridge of the trilobites and probably also anterior to the suture, as this is not complete at this stage. This white node reminds of the small facial ridge in Arionellus ceticephalus Barr.
[17] Packard, Development of Limulus polyphemus. Memoirs Boston Soc. Nat. H. vol. II, pl. V, fig. 25. Nothing is said about the exterior structure of the eyes.
Dohrn, Zur Embryologie and Morphologie des Limulus Polyphemus. Jenaische Zeitschrift 1871. A very good figure (pl. XIV, f. 4) shows clearly the two parts of the eyes, the interior one being larger.
Kingsley also (Devel. of Limulus, Journal of Morphology, vol. 7, 1892, pl. VI, fig. 34) has in the last larval stage the suture and the eye in two parts, of which one is white lying inside the suture and the eye proper, black, outside.
As Kishinouya[18] has already pointed out and as I have anticipated above the head of the most developed trilobites in their adult state, and the head of the larval Limulus consists of five parts, viz. 1) the glabella in the centre, 2 & 3) the fixed cheeks, 4 & 5) the free cheeks. An elevated ridge in the adult Limulus shows where the suture once lay and it is on the outside thereof that the eye of the adult is placed. What other authors call the ridge or the eye ridge in Limulus, Kishinouya rightly names a suture.
[18] Journal College of Science Tokio, vol. V, p. 53, 1892.
It was the renowned Swedish naturalist Wahlenberg, who first recognized the importance of the facial suture, which he called »linea ocularis»,[19] but to another Swedish palæontologist Dalman[20] the exact definition of this suture is due, to which he gave the name still in use. He expressly remarks that the suture crosses the ocular node and limits the outside of the »lobus palpebralis» and he makes a clear distinction between that lobe and the »tuberculi and eminentiæ oculares» (= facial ridge) of which he says that they are the more or less evident elevations situated in the blind Palæades on the place of the eyes (in which he is wrong) »and which perhaps are an indication of such organs». But then he says doubtfully (p. 255) »tuberculorum ocularium veram naturam determinare haud ausi sumus, etsi oculorum formam sat bene exhibere videantur» and he adds concerning Paradoxides »oculi nulli, eorum loco autem tuberculi duo».
[19] Additamenta quædam ad petrificata telluris Suecana, in Acta Upsaliensia, vol VIII, 1821, p. 294.
[20] Vet.-Akad. Handl. 1826 p. 126.
Group 2. The Olenidæ and related families.