106 Miss Agnes Crane — Evolution of the Brachiopoda. 



launched into the sea of life, perpetuated a race of individuals with 

 the same accelei'ated chai'acters and propensities (3). 



Furthermore, we discover as we proceed with our inquiries, that 

 such distinctions as are expressed in the class names Lyopomata or 

 " loose valves," comprising all the genera with inarticulated valves, 

 and the Arihropomata, or " attached valves," including all those 

 with articulated valves, are not in reality fundamental distinctions. 

 It is now known that some forms of Brachiopoda once existed which 

 fairly bridged the gulf, formerly considered impassable, between 

 these two natural groups of hinged and hingeless forms, and that in 

 inore than one direction. We may cite as examples, Neobolns, 

 Spondijloholus, Trematobolus, and Kutorgina of the Cambrian period, 

 Trimerella and Monomerella of the Silurian, and Barroisella of the 

 Devonian (Lingula suhspatulaia, M. and W., type), which indicate 

 different approximations between the hingeless and the hinged forms 

 (38). The fact, moreover, that Mr. G. F. Mathew (52) was com- 

 pelled to describe his new genus Trematobolus from the St. John 

 horizon in North America as an articulated Brachiopod of the order 

 Inarticulata, emphasizes this point in a very marked manner. 



Let us now briefly consider some of these recent genealogical 

 revelations. It will, I think, be generally admitted that each 

 distinct group of Brachiopoda is first represented by small species, 

 somewhat insignificant in size and number of genera, although 

 perhaps rich in species and individuals in favourable localities.^ 

 In 1877 Dall (26) and Davidson (24) estimated the number of 

 known genera at 130, dividing them respectively into eighteen and 

 fourteen family groups. In 1887 Fischer and D. and P. GEhlert (63) 

 admitted sixteen. This year Mr. Charles Schuchert (64) took a fresh 

 census, tabulated 279 genera as valid, and classed them into forty- 

 seven family and sub-family gi'oups. Two-thirds of these were 

 differentiated in the Palaeozoic era^ ranging from the Primordial to 

 the Carboniferous and Permian inclusive. But only nine of these 

 passed up into the Mesozoic, when, however, some fresh types 

 appeared which gave rise to a secondary evolution on fresh lines of 

 development. Six out of the seven surviving families were repre- 

 sented in the Palaeozoic. The genealogy of the Thecidiid^ is inter- 

 rupted at the Carbo-Permian epoch, but there is good reason to 

 believe that their remoter ancestors lived as long; ao;o as the Silurian. 



The Cuvierian class Bracliiopoda is still subdivided by Beecher (4) 

 and Schuchert (64), Fischer and the CEhlerts (63), into the sub- 

 classes Lyopomata and Arihropomata of Owen, as the distinction of 

 loose and attached valves is one which conveniently characterizes an 

 immense majority of the described forms. The sub-classes were again 

 subdivided into six orders by Waagen (68) in 1883, into eight by 

 Neumayr (58), and by Dr. Beecher (4) into four in 1891. His 

 classification (5) seems a ratioTial one, based on the nature of the 

 pedicle-passage and certain stages of shell growth. It embodies 

 some of the best features of the systems of Von Buch, Deslong- 

 champs (33, 34, 35), King (48), Gray, Davidson (28), Dall (25, 

 1 String ocephalus and Schaphioccelia are abnormal exceptions. ^ 



