An ontogenic series of 17 specimens (mainly dorsal exoskeletons) showing the gradual increase in the number of thoracic segments released from the transitory pygidium, from 2 to 22 (the full number for this genus). The series can be used to demonstrate the meraspid period of ontogeny, showing larval development "degree" 1 to "degree" 21, the latter being the start of the holaspid period. As most of the specimens are complete, the series can also be used to show changes in the form and relative proportions of individual parts of the cephalon and the relative proportions of the transitory pygidium. The use of a x8 or x10 hand lens or binocular microscope is essential to see the detail on the smaller members of the series. All specimens are on individual blocks which in most cases are approximately square and fall into the size range 20 mm to 40 mm. The individual member of the series are described, the suffix numbers in brackets () correspond to the number of thoracic segments. From Lodenice, Czech Rep.