142 INVERTEBRATE PALAEONTOLOGY 



Neotremata can be classed into four groups, two of 

 which were restricted to the Lower Palaeozoic. The 

 other two (Discinacea and Craniacea) did not appear 

 until the Ordovician, and, like the contemporaneous 

 Lingulids, have proved persistent, though showing their 

 greatest differentiation in pre-Devonian times. Acrotreta 

 is the best-known genus of Neotremata occurring in the 

 British Cambrian and Ordovician ; Orbiculoidea (PL ix. 

 fig. 9) is equally familiar in the Wenlock and Ludlow 

 stages of the Silurian. 



Very few Protremata occur in the Cambrian, but 

 archaic forms of Orthacea and Pentameracea are known. 

 Billingsella and Eoorthis are the only types certainly 

 recognized in Britain. In the Ordovician, the Orthidae 

 arose and reached their acme. Orthis (PL ix. fig. 10), 

 Plectorthis and Hebertella were abundant genera. Com- 

 parable acceleration was shown by the Strophomenidae, 

 Strophomena (PL ix. fig. n), Rafinesquina and Plectam- 

 bonites occurring in great profusion in some beds of the 

 Upper Ordovician. The first of the Chonetes stock, 

 from which the Upper Palaeozoic Productids were 

 evolved, made its appearance in this period. Small 

 Pentameracea, such as Clitambonites and Porambonites, 

 are locally abundant. In the Silurian, the Orthacea 

 and Strophomenacea continued the acmaic success 

 attained in the previous period, Plectorthis, Dalmanella 

 and Bilobites generously representing the former, and 

 Leptaena,) Strophonella, Streptis and Chonetes (PL ix. fig. 

 12) the latter. Pentamerids attained their acme in this 

 period ; the smooth Pentamerus forms beds of limestone 

 in the Llandovery, the partly plicate Gypidula abounds in 

 the Wenlock, and the large, evenly-ribbed Conchidium 

 (PL ii. fig. i) builds much of the Ludlow Limestone. 



Telotremata are the least important order of Lower 

 Palaeozoic Brachiopods ; and, were it not for the 



