A modification of Salter’s classification has been brought forward by Beecher[[200]] who divides the Trilobita into three main groups:—
1. Hypoparia. Facial sutures at or near the margin, or ventral. Compound eyes absent. This is equivalent to Salter’s Agnostini and Ampycini with the addition of the Harpedidae.
2. Opisthoparia. Facial sutures extending from the posterior margin to the front margin, but occasionally uniting in front of the glabella. Eyes holochroal or prismatic, but sometimes absent. This comprises the same families as Salter’s Asaphini with the exclusion of the Harpedidae and Calymenidae.
3. Proparia. Facial sutures extending from the lateral margins, and either cutting the anterior margin or uniting in front of the glabella. Eyes holochroal or schizochroal; occasionally absent. This is equivalent to Salter’s Phacopini with the addition of the Calymenidae.
In each of the groups proposed Beecher regards as the more primitive forms those which possess characters similar to those of the early larval stages, such as narrow free cheeks, the absence of compound eyes, and a glabella which is broad in front and reaches the anterior margin of the head.
The modifications introduced by Beecher can scarcely be regarded as making Salter’s classification more natural. For instance, the Agnostidae differ so much from all other families that, at present, there is no evidence to show that they have any close phylogenetic relationship with the Trinucleidae and Harpedidae. Further, the Calymenidae, which Salter recognised as related to the Olenidae, have been shown by the careful work of Professor Pompeckj[[201]] to have descended from the latter family, and to have no genetic connexion with the Phacopidae with which they are grouped by Beecher. Then in the Trinucleidae the earliest genus, Orometopus[[202]] (Fig. [140], A), possesses compound eyes and facial sutures which begin at the posterior margin and unite in front of the glabella; so that, according to Beecher’s classification, that genus would belong to the Opisthoparia, whereas the later genera (Trinucleus, etc.) of the same family would be placed in the Hypoparia. At present, therefore, the only classification of Trilobites which can be adopted is a division into families, of which a short account is given below.
Fig. [146].—Agnostus integer, Beyr., × 8. Cambrian. (After Barrande.)
Fam. 1. Agnostidae (Fig. [146]).—Small Trilobites, in which the head and pygidium are of nearly the same size and shape. The thorax is shorter than the head or pygidium, and consists of from two to four segments with grooved pleurae. The length and width of the head are commonly nearly equal, but sometimes the length is greater. Eyes are absent. Facial sutures appear to be absent, but are stated by Beecher to be at the margin of the cephalic shield. From the absence of eyes, the probable absence of facial sutures, the few or indistinct furrows on the glabella, and the smaller number of thoracic segments, the Agnostidae appear to be degenerate forms. Microdiscus is apparently less modified than Agnostus, on account of the larger number of thoracic segments, the more distinct segmentation of the pygidium, and, in some species, the larger number of furrows on the glabella. Cambrian and Ordovician. Genera: Agnostus, Microdiscus.
Fam. 2. Shumardiidae.—The body is very small and oval. The cephalic shield is nearly semicircular and very convex, with a broad glabella which expands in front, and in which the furrows, except the neck-furrow, are indistinct. The facial suture is marginal and eyes are absent. There are six thoracic segments with ridged pleurae; the axis is broader than the pleurae. The pygidium is large, and is formed of about four segments similar to those of the thorax. Upper Cambrian and Ordovician. Genus: Shumardia.