the so-called Latinus stone of Whithorn, dating to c. 450. In the east and north, Class II Pictish stones began to show Christian symbolism from the early eighth century. Early church buildings may originally have been wooden, like that excavated at Whithorn, but of those for which evidence survives from this era are basic masonry-built churches, beginning on the west coast and islands and spreading south and east. Early chapels tended to have square ended converging walls, similar to Irish chapels of this period. Medieval parish church architecture in Scotland was typically much less elaborate than in England, with many churches remaining simple oblongs, without transepts and aisles, and often without towers. In the Highlands they were often even simpler, many built of rubble masonry and sometimes indistinguishable from the outside from houses or farm buildings. Celtic Christianity The Celtic Church is a term that has been used by scholars to describe a specific form of Christianity with its origins in the conversion of Ireland, traditionally associated with St. Patrick. This form of Christianity later spread to northern Britain through Iona. It is also used as a general description for the Christian establishment of northern Britain prior to the twelfth century, when new religious institutions and ideologies of primarily French origin began to take root in Scotland. The Celtic form of Christianity has been contrasted with that derived from missions from Rome, which reached southern England in 587 under the leadership of St. Augustine of Canterbury. Subsequent missions from Canterbury then helped convert the Anglo-Saxon kingdoms, reaching Northumbria in the early eighth century, where Iona had already begun to have a presence. As a result, Christianity in Northumbria became a mix of Celtic and Roman influences. While Roman and Celtic Christianity were very similar in doctrine and both accepted ultimate papal authority, there were differences in practice. The most contentious were the method of calculating Easter, and the form of head shaving for priests known as tonsure. Other differences were in the rites of ordination and baptism, and in the form of service of the liturgy. In addition scholars have identified significant characteristics of the organisation of Irish and Scottish Christianity as relaxed ideas of clerical celibacy, intense secularisation of ecclesiastical institutions, and the lack of a diocesan structure. This made abbots (or coarbs), rather than bishops, the most important element the church hierarchy. In the seventh century the Northumbrian